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

Gallstone and Gallbladder Disease: Biliary Tract and Cholangiopathies

Cholestatic liver diseases are named primarily due to the blockage of bile flow and buildup of bile acids in the liver. Cholestasis can occur in cholangiopathies, fatty liver diseases, and during COVID-19 infection. Most literature evaluates damage occurring to the intrahepatic biliary tree during cholestasis; however, there may be associations between liver damage and gallbladder damage. Gallbladder damage can manifest as acute or chronic inflammation, perforation, polyps, cancer, and most commonly gallstones. Considering the gallbladder is an extension of the intrahepatic biliary network, and both tissues are lined by biliary epithelial cells that share common mechanisms and properties, it is worth further evaluation to understand the association between bile duct and gallbladder damage. In this comprehensive article, we discuss background information of the biliary tree and gallbladder, from function, damage, and therapeutic approaches. We then discuss published findings that identify gallbladder disorders in various liver diseases. Lastly, we provide the clinical aspect of gallbladder disorders in liver diseases and ways to enhance diagnostic and therapeutic approaches for congruent diagnosis. © 2023 American Physiological Society. Compr Physiol 13:4909-4943, 2023.

Distinct Alteration of Gene Expression Programs in the Small Intestine of Male and Female Mice in Response to Ablation of Intestinal Fabp Genes

Fatty acid-binding proteins (Fabps) make up a family of widely distributed cytoplasmic lipid-binding proteins. The small intestine contains three predominant Fabp species, Fabp1, Fabp2, and Fabp6. Our previous studies showed that Fabp2 and Fabp6 gene-disrupted mice exhibited sexually dimorphic phenotypes. In this study, we carried out a systematic comparative analysis of the small intestinal transcriptomes of 10 week-old wild-type (WT) and Fabp gene-disrupted male and female mice. We found that the small intestinal transcriptome of male and female mice showed key differences in the gene expression profiles that affect major biological processes. The deletion of specific Fabp genes induced unique and sex-specific changes in the gene expression program, although some differentially expressed genes in certain genotypes were common to both sexes. Functional annotation and interaction network analyses revealed that the number and type of affected pathways, as well as the sets of interacting nodes in each of the Fabp genotypes, are partitioned by sex. To our knowledge, this is the first time that sex differences were identified and categorized at the transcriptome level in mice lacking different intestinal Fabps. The distinctive transcriptome profiles of WT male and female small intestine may predetermine the nature of transcriptional reprogramming that manifests as sexually dimorphic responses to the ablation of intestinal Fabp genes.

The effect of ursodeoxycholic acid on the relative expression of the lipid metabolism genes in mouse cholesterol gallstone models

BACKGROUND

Many studies indicate that gallstone formation has genetic components. The abnormal expression of lipid-related genes could be the basis for particular forms of cholesterol gallstone disease. The aim of this study was to obtain insight into lipid metabolism disorder during cholesterol gallstone formation and to evaluate the effect of ursodeoxycholic acid (UDCA) on the improvement of bile lithogenicity and its potential influence on the transcription of lipid-related genes.

METHODS

Gallstone-susceptible mouse models were induced by feeding with a lithogenic diet (LD) for 8 weeks. Bile and liver tissues were obtained from these mouse models after 0, 4 and 8 weeks. Bile lipids were measured enzymatically, and the cholesterol saturation index (CSI) was calculated to evaluate the bile lithogenicity by using Carey's critical tables. Real-time polymerase chain reaction (RT-PCR) was used to detect the mRNA expression levels of farnesoid X receptor (FXR), liver X receptor (LXR), adenosine triphosphate-binding cassette subfamily G member 5/8 (ABCG5/8), cholesterol 7-α hydroxylase (CYP7A1), oxysterol 7-α hydroxylase (CYP7B1), sterol 27-α hydroxylase (CYP27A1), peroxisome proliferator-activated receptor alpha (PPAR-α) and adenosine triphosphate-binding cassette subfamily B member 11 (ABCB11).

RESULTS

The rate of gallstone formation was 100% in the 4-week group but only 30% in the UDCA-treated group. The UDCA-treated group had a significantly lower CSI compared with other groups. Of special note, the data on the effects of UDCA showed higher expression levels of ABCG8, ABCB11 and CYP27A1, as well as lower expression levels of LXR and PPAR-α, compared to the model control group.

CONCLUSIONS

UDCA exhibits tremendously potent activity in restraining lipid accumulation, thus reversing the lithogenic effect and protecting hepatocytes from serious pathological damage. The abnormal expression of ABCG8, CYP7A1, CYP27A1, LXR and PPAR-α might lead to high lithogenicity of bile. These results are helpful in exploring new lipid metabolism pathways and potential targets for the treatment of cholesterol stones and for providing some basis for the study of the pathogenesis and genetic characteristics of cholelithiasis. Research on the mechanism of UDCA in improving lipid metabolism and bile lithogenicity may be helpful for clinical treatment and for reducing the incidence of gallstones.

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.

Mouse models of gallstone disease

PURPOSE OF REVIEW

The establishment of mouse models of gallstones, and the contribution of mouse models to genetic studies of gallstone disease, as well as the latest advances in the pathophysiology of gallstones from mouse experiments are summarized.

RECENT FINDINGS

The combined uses of genomic strategies and phenotypic studies in mice have successfully led to the identification of many Lith genes, which pave the way for the discovery of human LITH genes. The physical-chemical, genetic, and molecular biological studies of gallstone disease in mice with knockout or transgene of specific target genes have provided many novel insights into the complex pathophysiological mechanisms of this very common hepatobiliary disease worldwide, showing that interactions of five primary defects play a critical role in the pathogenesis of cholesterol gallstones. Based on mouse studies, a new concept has been proposed that hepatic hypersecretion of biliary cholesterol is induced by multiple Lith genes, with insulin resistance as part of the metabolic syndrome interacting with cholelithogenic environmental factors to cause the phenotype.

SUMMARY

The mouse model of gallstones is crucial for elucidating the physical-chemical and genetic mechanisms of cholesterol crystallization and gallstone formation, which greatly increase our understanding of the pathogenesis of this disease in humans.

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

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

Cd36 knockout mice are protected against lithogenic diet-induced gallstones

The scavenger receptor and multiligand transporter CD36 functions to promote cellular free fatty acid uptake and regulates aspects of both hepatic and intestinal cholesterol metabolism. However, the role of CD36 in regulating canalicular and biliary cholesterol transport and secretion is unknown. Here, we show that germline Cd36 knockout (KO) mice are protected against lithogenic diet (LD)-induced gallstones compared with congenic (C57BL6/J) controls. Cd36 KO mice crossed into congenic L-Fabp KO mice (DKO mice) demonstrated protection against LD-induced gallstones, reversing the susceptibility phenotype observed in L-Fabp KO mice. DKO mice demonstrated reduced biliary cholesterol secretion and a shift into more hydrophophilic bile acid species, without changes in either BA pool size or fecal excretion. In addition, we found that the mean and maximum force of gallbladder contraction was increased in germline Cd36 KO mice, and gallbladder lipid content was reduced compared with wild-type controls. Finally, whereas germline Cd36 KO mice were protected against LD-induced gallstones, neither liver- nor intestine-specific Cd36 KO mice were protected. Taken together, our findings show that CD36 plays an important role in modifying gallstone susceptibility in mice, at least in part by altering biliary lipid composition, but also by promoting gallbladder contractility.

Molecular dynamics simulations of apo and holo forms of fatty acid binding protein 5 and cellular retinoic acid binding protein II reveal highly mobile protein, retinoic acid ligand, and water molecules

Structural and dynamic properties from a series of 300 ns molecular dynamics, MD, simulations of two intracellular lipid binding proteins, iLBPs, (Fatty Acid Binding Protein 5, FABP5, and Cellular Retinoic Acid Binding Protein II, CRABP-II) in both the apo form and when bound with retinoic acid reveal a high degree of protein and ligand flexibility. The ratio of FABP5 to CRABP-II in a cell may determine whether it undergoes natural apoptosis or unrestricted cell growth in the presence of retinoic acid. As a result, FABP5 is a promising target for cancer therapy. The MD simulations presented here reveal distinct differences in the two proteins and provide insight into the binding mechanism. CRABP-II is a much larger, more flexible protein that closes upon ligand binding, where FABP5 transitions to an open state in the holo form. The traditional understanding obtained from crystal structures of the gap between two β-sheets of the β-barrel common to iLBPs and the α-helix cap that forms the portal to the binding pocket is insufficient for describing protein conformation (open vs. closed) or ligand entry and exit. When the high degree of mobility between multiple conformations of both the ligand and protein are examined via MD simulation, a new mode of ligand motion that improves understanding of binding dynamics is revealed.

Intracellular cholesterol transport proteins enhance hydrolysis of HDL-CEs and facilitate elimination of cholesterol into bile

While HDL-associated unesterified or free cholesterol (FC) is thought to be rapidly secreted into the bile, the fate of HDL-associated cholesteryl esters (HDL-CEs) that represent >80% of HDL-cholesterol, is only beginning to be understood. In the present study, we examined the hypothesis that intracellular cholesterol transport proteins [sterol carrier protein 2 (SCP2) and fatty acid binding protein-1 (FABP1)] not only facilitate CE hydrolase-mediated hydrolysis of HDL-CEs, but also enhance elimination of cholesterol into bile. Adenovirus-mediated overexpression of FABP1 or SCP2 in primary hepatocytes significantly increased hydrolysis of HDL-[(3)H]CE, reduced resecretion of HDL-CE-derived FC as nascent HDL, and increased its secretion as bile acids. Consistently, the flux of [(3)H]cholesterol from HDL-[(3)H]CE to biliary bile acids was increased by overexpression of SCP2 or FABP1 in vivo and reduced in SCP2(-/-) mice. Increased flux of HDL-[(3)H]CE to biliary FC was noted with FABP1 overexpression and in SCP2(-/-) mice that have increased FABP1 expression. Lack of a significant decrease in the flux of HDL-[(3)H]CE to biliary FC or bile acids in FABP1(-/-) mice indicates the likely compensation of its function by an as yet unidentified mechanism. Taken together, these studies demonstrate that FABP1 and SCP2 facilitate the preferential movement of HDL-CEs to bile for final elimination.

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.

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

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

The unique ligand binding features of subfamily-II iLBPs with respect to bile salts and related drugs

Intracellular lipid binding proteins (iLBPs) are a family of evolutionarily related small cytoplasmic proteins implicated in the transcellular transport of lipophilic ligands. Subfamily-II iLBPs include the liver fatty acid binding protein (L-FABP), and the ileal and the liver and ileal bile acid binding proteins (L-BABP and I-BABP). Atomic-level investigations during the past 15-20 years have delivered relevant information on bile acid binding by this protein group, revealing unique features including binding cooperativity, promiscuity, and site selectivity. Using NMR spectroscopy and other biophysical techniques, our laboratories have contributed to an understanding of the molecular determinants of some of these properties and their generality among proteins from different animal species. We focused especially on formation of heterotypic complexes, considering the mixed compositions of physiological bile acid pools. Experiments performed with synthetic bile acid derivatives showed that iLBPs could act as targets for cell-specific contrast agents and, more generally, as effective carriers of amphiphilic drugs. This review collects the major findings related to bile salt interactions with iLBPs aiming to provide keys for a deeper understanding of protein-mediated intracellular bile salt trafficking.

Bile salt recognition by human liver fatty acid binding protein

Fatty acid binding proteins (FABPs) act as intracellular carriers of lipid molecules, and play a role in global metabolism regulation. Liver FABP (L-FABP) is prominent among FABPs for its wide ligand repertoire, which includes long-chain fatty acids as well as bile acids (BAs). In this work, we performed a detailed molecular- and atomic-level analysis of the interactions established by human L-FABP with nine BAs to understand the binding specificity for this important class of cholesterol-derived metabolites. Protein-ligand complex formation was monitored using heteronuclear NMR, steady-state fluorescence spectroscopy, and mass spectrometry. BAs were found to interact with L-FABP with dissociation constants in the narrow range of 0.6-7 μm; however, the diverse substitution patterns of the sterol nucleus and the presence of side-chain conjugation resulted in complexes endowed with various degrees of conformational heterogeneity. Trihydroxylated BAs formed monomeric complexes in which single ligand molecules occupied similar internal binding sites, based on chemical-shift perturbation data. Analysis of NMR line shapes upon progressive addition of taurocholate indicated that the binding mechanism departed from a simple binary association equilibrium, and instead involved intermediates along the binding path. The co-linear chemical shift behavior observed for L-FABP complexes with cholate derivatives added insight into conformational dynamics in the presence of ligands. The observed spectroscopic features of L-FABP/BA complexes, discussed in relation to ligand chemistry, suggest possible molecular determinants of recognition, with implications regarding intracellular BA transport. Our findings suggest that human L-FABP is a poorly selective, universal BA binder.

Ablating L-FABP in SCP-2/SCP-x null mice impairs bile acid metabolism and biliary HDL-cholesterol secretion

On the basis of their abilities to bind bile acids and/or cholesterol, the physiological role(s) of liver fatty acid-binding protein (L-FABP) and sterol carrier protein (SCP) 2/SCP-x (SCP-2/SCP-x) gene products in biliary bile acid and cholesterol formation was examined in gene-ablated male mice. L-FABP (LKO) or L-FABP/SCP-2/SCP-x [triple-knockout (TKO)] ablation markedly decreased hepatic bile acid concentration, while SCP-2/SCP-x [double-knockout (DKO)] ablation alone had no effect. In contrast, LKO increased biliary bile acid, while DKO and TKO had no effect on biliary bile acid levels. LKO and DKO also altered biliary bile acid composition to increase bile acid hydrophobicity. Furthermore, LKO and TKO decreased hepatic uptake and biliary secretion of high-density lipoprotein (HDL)-derived 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3β-ol (NBD-cholesterol), while DKO alone had no effect. Finally, LKO and, to a lesser extent, DKO decreased most indexes contributing to cholesterol solubility in biliary bile. These results suggest different, but complementary, roles for L-FABP and SCP-2/SCP-x in biliary bile acid and cholesterol formation. L-FABP appears to function more in hepatic retention of bile acids as well as hepatic uptake and biliary secretion of HDL-cholesterol. Conversely, SCP-2/SCP-x may function more in formation and biliary secretion of bile acid, with less impact on hepatic uptake or biliary secretion of HDL-cholesterol.

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

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

The number of metabolic abnormalities associated with the risk of gallstones in a non-diabetic population

AIM

To evaluate whether metabolic syndrome is associated with gallstones, independent of hepatitis C infection or chronic kidney disease (CKD), in a non-diabetic population.

MATERIALS AND METHODS

A total of 8,188 Chinese adult participants that underwent a self-motivated health examination were recruited into the final analysis after excluding the subjects who had a history of cholecystectomy, diabetes mellitus, or were currently using antihypertensive or lipid-lowering agents. Gallstones were defined by the presence of strong intraluminal echoes that were gravity-dependent or that attenuated ultrasound transmission.

RESULTS

A total of 447 subjects (5.5%) had gallstones, with 239 (5.1%) men and 208 (6.0%) women. After adjusting for age, gender, obesity, education level, and lifestyle factors, included current smoking, alcohol drinking, regular exercise, hepatitis B, hepatitis C, and CKD, there was a positive association between metabolic syndrome and gallstones. Moreover, as compared to subjects without metabolic abnormalities, subjects with one, two, and three or more suffered from a 35, 40, and 59% higher risk of gallstones, respectively.

CONCLUSIONS

Non-diabetic subjects with metabolic syndrome had a higher risk of gallstones independent of hepatitis C or CKD, and a dose-dependent effect of metabolic abnormalities also exists.

Hepatic Mttp deletion reverses gallstone susceptibility in L-Fabp knockout mice

Previous studies demonstrated that L-Fabp KO mice are more susceptible to lithogenic diet (LD)-induced gallstones because of altered hepatic cholesterol metabolism and increased canalicular cholesterol secretion. Other studies demonstrated that liver-specific deletion of microsomal triglyceride transfer protein (Mttp-LKO) reduced LD-induced gallstone formation by increasing biliary phospholipid secretion. Here we show that mice with combined deletion (i.e., DKO mice) are protected from LD-induced gallstone formation. Following 2 weeks of LD feeding, 73% of WT and 100% of L-Fabp KO mice developed gallstones versus 18% of Mttp-LKO and 23% of DKO mice. This phenotype was recapitulated in both WT and L-Fabp KO mice treated with an Mttp antisense oligonucleotide (M-ASO). Biliary cholesterol secretion was increased in LD-fed L-Fabp KO mice and decreased in DKO mice. However, phospholipid secretion was unchanged in LD-fed Mttp-LKO and DKO mice as well as in M-ASO-treated mice. Expression of the canalicular export pump ABCG5/G8 was reduced in LD-fed DKO mice and in M-ASO-treated L-Fabp KO mice. We conclude that liver-specific Mttp deletion not only eliminates apical lipoprotein secretion from hepatocytes but also attenuates canalicular cholesterol secretion, which in turn decreases LD-induced gallstone susceptibility.

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.

Serum adipocyte fatty acid-binding protein levels are independently associated with sarcopenic obesity

AIMS

Adipocyte fatty acid-binding protein (A-FABP) plays a key role in obesity-related insulin resistance and inflammation which may be involved in the pathogenesis of sarcopenic obesity (SO). This study examined the association of SO with serum A-FABP levels in Korean adults.

METHODS

Two hundred ninety eight adults aged 20-70 years were examined using dual X-ray absorptiometry and computed tomography and measuring serum A-FABP levels. Sarcopenia was defined as the appendicular skeletal muscle mass (ASM) divided by weight (%) of <1 SD below the mean values of young adults in both sexes. Obesity was defined as visceral fat area (VFA) ≥ 100 cm(2).

RESULTS

Serum A-FABP levels were higher in groups with SO compared to non-SO groups in both men and women. In the unadjusted model, serum A-FABP levels were positively associated with VFA and negatively associated with ASM/weight. Even after adjusting for possible confounding factors, ASM/weight was found to be independently and negatively associated with serum A-FABP levels. In addition, multiple logistic regression analysis showed that increased serum levels of A-FABP were independently associated with the presence of SO.

CONCLUSIONS

The present findings indicate that serum A-FABP levels may be valuable markers of the presence of SO.

Transport and biological activities of bile acids

Bile acids have emerged as important biological molecules that support the solubilization of various lipids and lipid-soluble compounds in the gut, and the regulation of gene expression and cellular function. Bile acids are synthesized from cholesterol in the liver and eventually released into the small intestine. The majority of bile acids are recovered in the distal end of the small intestine and then returned to the liver for reuse. The components of the mechanism responsible for the recycling of bile acids within the enterohepatic circulation have been identified whereas the mechanism for intracellular transport is less understood. Recently, the ileal lipid binding protein (ILBP; human gene symbol FABP6) was shown to be needed for the efficient transport of bile acids from the apical side to the basolateral side of enterocytes in the distal intestine. This review presents an overview of the transport of bile acids between the liver and the gut as well as within hepatocytes and enterocytes. A variety of pathologies is associated with the malfunction of the bile acid transport system.

Monitoring bile acid transport in single living cells using a genetically encoded Förster resonance energy transfer sensor

Bile acids are pivotal for the absorption of dietary lipids and vitamins and function as important signaling molecules in metabolism. Here, we describe a genetically encoded fluorescent bile acid sensor (BAS) that allows for spatiotemporal monitoring of bile acid transport in single living cells. Changes in concentration of multiple physiological and pathophysiological bile acid species were detected as robust changes in Förster resonance energy transfer (FRET) in a range of cell types. Specific subcellular targeting of the sensor demonstrated rapid influx of bile acids into the cytoplasm and nucleus, but no FRET changes were observed in the peroxisomes. Furthermore, expression of the liver fatty acid binding protein reduced the availability of bile acids in the nucleus. The sensor allows for single cell visualization of uptake and accumulation of conjugated bile acids, mediated by the Na(+)-taurocholate cotransporting protein (NTCP). In addition, cyprinol sulphate uptake, mediated by the putative zebrafish homologue of the apical sodium bile acid transporter, was visualized using a sensor based on the zebrafish farnesoid X receptor. The reversible nature of the sensor also enabled measurements of bile acid efflux in living cells, and expression of the organic solute transporter αβ (OSTαβ) resulted in influx and efflux of conjugated chenodeoxycholic acid. Finally, combined visualization of bile acid uptake and fluorescent labeling of several NTCP variants indicated that the sensor can also be used to study the functional effect of patient mutations in genes affecting bile acid homeostasis.

CONCLUSION

A genetically encoded fluorescent BAS was developed that allows intracellular imaging of bile acid homeostasis in single living cells in real time.

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.

Gallstone disease is associated with more severe liver damage in patients with non-alcoholic fatty liver disease

Background

Nonalcoholic fatty liver disease (NAFLD) and gallstone disease (GD) are both highly prevalent in the general population and associated with obesity and insulin resistance. We aimed to evaluate the prevalence of GD in a cross sectional study of NAFLD patients and to define whether the presence of GD is associated with diabetes and predicts more severe liver disease.

Methodology/Principal Findings

We merged databases of four Liver Units, comprising 524 consecutive biopsy-proven NAFLD (373 males) observed between January 2003 and June 2010. GD was diagnosed in 108 (20%), and 313 cases (60%) were classified by liver biopsy as nonalcoholic steatohepatitis (NASH). The GD subgroup was characterized by a significantly higher prevalence of females, prediabetes/diabetes, abdominal obesity and metabolic syndrome, older age, higher BMI, fasting glucose, HOMA-IR and lower ALT. The prevalence of GD progressively increased with advancing fibrosis and with the severity of necroinflammatory activity (p for trend  = 0.0001 and  = 0.01, respectively), without differences in the severity of steatosis. At multivariate analysis GD was associated with female gender (OR 1.37, 95% CI 1.04–1.8), age (OR 1.027, 95% CI1.003–1.05), fasting glucose (OR 1.21, 95% CI 1.10–1.33) and NASH (OR 1.40,95% CI 1.06–1.89), whereas ALT levels were associated with a lower GD risk (OR 0.98, 95% CI 0.97–0.99). When subjects with cirrhosis were excluded from analysis, the association between GD and fasting glucose, female gender, and NASH was maintained.

Conclusion

Patients with NAFLD have a high prevalence of GD, which characterizes subjects with altered glucose regulation and more advanced liver disease.

Effects of SLC10A2 variant rs9514089 on gallstone risk and serum cholesterol levels- meta-analysis of three independent cohorts

Background

Recently, a single nucleotide polymorphism (SNP) rs9514089 in SLC10A2 (apical sodium-dependent bile acid transporter gene) has been identified as a susceptibility variant for cholelithiasis in humans.

Methods

Here we assessed the effects of rs9514089 on gallstone risk and related phenotypes of the metabolic syndrome in the self-contained population of Sorbs (183 cases with gallstones/826 controls). Furthermore, we performed a meta-analysis for effects of rs9514089 on susceptibility for cholelithiasis in three independent cohorts (Stuttgart: 56 cases/71 controls, Aachen: 184 cases/184 controls and Sorbs).

Results

There was no significant association of rs9514089 with gallstone risk, serum lipid parameters and BMI in the Sorbs and in the meta-analysis of all three cohorts (p > 0.05). There was an effect trend in the subgroup of lean subjects but based on different effect directions in the three cohorts there was no significant association in the meta-analysis.

Conclusions

We were not able to replicate the effect of rs9514089 on gallstone risk in the Sorbs. Further analyses in larger cohorts are required to finally assess the role of genetic variants in SLC10A2 in human gallstone development and lipid metabolism.

APOE and FABP2 Polymorphisms and History of Myocardial Infarction, Stroke, Diabetes, and Gallbladder Disease

Dysfunctional lipid metabolism plays a central role in pathogenesis of major chronic diseases, and genetic factors are important determinants of individual lipid profiles. We analyzed the associations of two well-established functional polymorphisms (FABP2 A54T and APOE isoforms) with past and family histories of 1492 population samples. FABP2-T54 allele was associated with an increased risk of past history of myocardial infarction (odds ratio (OR) = 1.51). Likewise, the subjects with APOE4, compared with E2 and E3, had a significantly increased risk of past history myocardial infarction (OR = 1.89). The OR associated with APOE4 was specifically increased in women for past history of myocardial infarction but decreased for gallstone disease. Interactions between gender and APOE isoforms were also significant or marginally significant for these two conditions. FABP2-T54 allele may be a potential genetic marker for myocardial infarction, and APOE4 may exert sex-dependent effects on myocardial infarction and gallbladder disease.

Nonalcoholic fatty liver disease is associated with an altered hepatocyte microRNA profile in LDL receptor knockout mice

MicroRNAs modulate processes associated with cell cycle control and differentiation. Here we explored the potential of microRNAs in the modulation of hepatic lipid metabolism and the development of nonalcoholic fatty liver disease. MicroRNA profiles of hepatocytes from low-density lipoprotein (LDL) receptor knockout mice fed a chow diet or a hypertriglyceridemia/fatty liver-inducing Western-type diet (WTD) were determined using quantitative real-time polymerase chain reaction. Ninety-seven of 103 microRNAs measured were expressed by hepatocytes and low variability between hepatocyte pools was observed. Feeding WTD coincided with a marked fivefold decrease in the relative expression level of miR-216 (P<.05) and miR-302a (P<.01). Interestingly, an increased hepatic miR-216 expression was detected in response to fasting. MicroRNA/biological function linkage analysis suggested that the change in hepatocyte microRNA profiles in response to high dietary lipid levels is associated with changes in cell cycle control and proliferation. In accordance with a diminished miR-302a expression on the WTD, hepatocyte mRNA expression levels of miR-302a target genes ABCA1 and in particular ELOVL6 were increased in response to WTD (twofold to ninefold). This suggests a role for miR-302a in hepatic cholesterol, fatty acid and glucose metabolism. In conclusion, we have shown that fatty liver development in LDL receptor knockout mice is associated with a significant change in the hepatocyte microRNA profile, i.e., a fivefold decrease in miR-216 and miR-302a expression. Based upon our comparative gene and microRNA expression studies it is anticipated that miR-302a may prove to be a valuable therapeutic target in the regulation of hepatic fatty acid utilization and insulin resistance.

Disruption of the murine protein kinase Cbeta gene promotes gallstone formation and alters biliary lipid and hepatic cholesterol metabolism

The protein kinase C (PKC) family of Ca(2+) and/or lipid-activated serine-threonine protein kinases is implicated in the pathogenesis of obesity and insulin resistance. We recently reported that protein kinase Cβ (PKCβ), a calcium-, diacylglycerol-, and phospholipid-dependent kinase, is critical for maintaining whole body triglyceride homeostasis. We now report that PKCβ deficiency has profound effects on murine hepatic cholesterol metabolism, including hypersensitivity to diet-induced gallstone formation. The incidence of gallstones increased from 9% in control mice to 95% in PKCβ(-/-) mice. Gallstone formation in the mutant mice was accompanied by hyposecretion of bile acids with no alteration in fecal bile acid excretion, increased biliary cholesterol saturation and hydrophobicity indices, as well as hepatic p42/44(MAPK) activation, all of which enhance susceptibility to gallstone formation. Lithogenic diet-fed PKCβ(-/-) mice also displayed decreased expression of hepatic cholesterol-7α-hydroxylase (CYP7A1) and sterol 12α-hydroxylase (CYP8b1). Finally, feeding a modified lithogenic diet supplemented with milk fat, instead of cocoa butter, both increased the severity of and shortened the interval for gallstone formation in PKCβ(-/-) mice and was associated with dramatic increases in cholesterol saturation and hydrophobicity indices. Taken together, the findings reveal a hitherto unrecognized role of PKCβ in fine tuning diet-induced cholesterol and bile acid homeostasis, thus identifying PKCβ as a major physiological regulator of both triglyceride and cholesterol homeostasis.

Proliferative and nonproliferative lesions of the rat and mouse hepatobiliary system

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally-accepted nomenclature for proliferative and non-proliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature and differential diagnosis for classifying microscopic lesions observed in the hepatobiliary system of laboratory rats and mice, with color microphotographs illustrating examples of some lesions. The standardized nomenclature presented in this document is also available for society members electronically on the internet (http://goreni.org). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous and aging lesions as well as lesions induced by exposure to test materials. A widely accepted and utilized international harmonization of nomenclature for lesions of the hepatobiliary system in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.

Liver fatty acid-binding protein and obesity

While low levels of unesterified long chain fatty acids (LCFAs) are normal metabolic intermediates of dietary and endogenous fat, LCFAs are also potent regulators of key receptors/enzymes and at high levels become toxic detergents within the cell. Elevated levels of LCFAs are associated with diabetes, obesity and metabolic syndrome. Consequently, mammals evolved fatty acid-binding proteins (FABPs) that bind/sequester these potentially toxic free fatty acids in the cytosol and present them for rapid removal in oxidative (mitochondria, peroxisomes) or storage (endoplasmic reticulum, lipid droplets) organelles. Mammals have a large (15-member) family of FABPs with multiple members occurring within a single cell type. The first described FABP, liver-FABP (L-FABP or FABP1), is expressed in very high levels (2-5% of cytosolic protein) in liver as well as in intestine and kidney. Since L-FABP facilitates uptake and metabolism of LCFAs in vitro and in cultured cells, it was expected that abnormal function or loss of L-FABP would reduce hepatic LCFA uptake/oxidation and thereby increase LCFAs available for oxidation in muscle and/or storage in adipose. This prediction was confirmed in vitro with isolated liver slices and cultured primary hepatocytes from L-FABP gene-ablated mice. Despite unaltered food consumption when fed a control diet ad libitum, the L-FABP null mice exhibited age- and sex-dependent weight gain and increased fat tissue mass. The obese phenotype was exacerbated in L-FABP null mice pair fed a high-fat diet. Taken together with other findings, these data suggest that L-FABP could have an important role in preventing age- or diet-induced obesity.

Tissue-specific functions in the fatty acid-binding protein family

The intracellular fatty acid-binding proteins (FABPs) are abundantly expressed in almost all tissues. They exhibit high affinity binding of a single long-chain fatty acid, with the exception of liver FABP, which binds two fatty acids or other hydrophobic molecules. FABPs have highly similar tertiary structures consisting of a 10-stranded antiparallel β-barrel and an N-terminal helix-turn-helix motif. Research emerging in the last decade has suggested that FABPs have tissue-specific functions that reflect tissue-specific aspects of lipid and fatty acid metabolism. Proposed roles for FABPs include assimilation of dietary lipids in the intestine, targeting of liver lipids to catabolic and anabolic pathways, regulation of lipid storage and lipid-mediated gene expression in adipose tissue and macrophages, fatty acid targeting to β-oxidation pathways in muscle, and maintenance of phospholipid membranes in neural tissues. The regulation of these diverse processes is accompanied by the expression of different and sometimes multiple FABPs in these tissues and may be driven by protein-protein and protein-membrane interactions.

Effect of sterol carrier protein-2 gene ablation on HDL-mediated cholesterol efflux from cultured primary mouse hepatocytes

Although HDL-mediated cholesterol transport to the liver is well studied, cholesterol efflux from hepatocytes back to HDL is less well understood. Real-time imaging of efflux of 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol), which is poorly esterified, and [(3)H]cholesterol, which is extensively esterified, from cultured primary hepatocytes of wild-type and sterol carrier protein-2 (SCP-2) gene-ablated mice showed that 1) NBD-cholesterol efflux was affected by the type of lipoprotein acceptor, i.e., HDL3 over HDL2; 2) NBD-cholesterol efflux was rapid (detected in 1-2 min) and resolved into fast [half time (t((1/2))) = 2.4 min, 6% of total] and slow (t((1/2)) = 26.5 min, 94% of total) pools, consistent with protein- and vesicle-mediated cholesterol transfer, respectively; 3) SCP-2 gene ablation increased efflux of NBD-cholesterol, as well as [(3)H]cholesterol, albeit less so due to competition by esterification of [(3)H]cholesterol, but not NBD-cholesterol; and 4) SCP-2 gene ablation increased initial rate (2.3-fold) and size (9.7-fold) of rapid effluxing sterol, suggesting an increased contribution of molecular cholesterol transfer. In addition, colocalization, double-immunolabeling fluorescence resonance energy transfer, and electron microscopy, as well as cross-linking coimmunoprecipitation, indicated that SCP-2 directly interacted with the HDL receptor, scavenger receptor class B type 1 (SRB1), in hepatocytes. Other membrane proteins in cholesterol efflux [SRB1 and ATP-binding cassettes (ABC) A-1, ABCG-1, ABCG-5, and ABCG-8] and several soluble/vesicle-associated proteins facilitating intracellular cholesterol trafficking (StARDs, NPCs, ORPs) were not upregulated. However, loss of SCP-2 elicited twofold upregulation of liver fatty acid-binding protein (L-FABP), a protein with lower affinity for cholesterol but higher cytosolic concentration than SCP-2. Ablation of SCP-2 and L-FABP decreased HDL-mediated NBD-cholesterol efflux. These results indicate that SCP-2 expression plays a significant role in HDL-mediated cholesterol efflux by regulating the size of rapid vs. slow cholesterol efflux pools and/or eliciting concomitant upregulation of L-FABP in cultured primary hepatocytes.

Lith genes and genetic analysis of cholesterol gallstone formation

Epidemiologic investigations, clinical observations, and family and twin studies in humans, as well as gallstone prevalence investigations in inbred mouse models, support the concept that cholesterol cholelithiasis could result from a complex interaction of environmental factors and the effects of multiple undetermined genes. Quantitative trait locus (QTL) analysis is a powerful genetic method for identifying primary rate-limiting genetic defects and discriminating them from secondary downstream lithogenic effects caused by mutations of the primary genes, and the subsequent positional cloning of such genes responsible for QTLs, followed by the use of manufactured mouse strains with "knockout" or "knockin" of the genes, could lead to the discovery of lithogenic actions of gallstone (LITH) genes. The combined use of genomic strategies and phenotypic studies in inbred strains of mice has successfully resulted in the identification of many candidate LITH genes. Because there is exceptionally close homology between mouse and human genomes, the orthologous human LITH genes can be identified from the mouse study. The discovery of LITH genes and more fundamental knowledge concerning the genetic determinants and molecular mechanisms underlying the formation of cholesterol gallstones in humans will pave the way for critical diagnostic and prelithogenic preventive measures for this exceptionally prevalent digestive 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.

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.