Contribution of vesicular and micellar carriers to cholesterol transport in human bile

Recent Advances in the Critical Role of the Sterol Efflux Transporters ABCG5/G8 in Health and Disease

Cardiovascular disease is characterized by lipid accumulation, inflammatory response, cell death, and fibrosis in the arterial wall and is the leading cause of morbidity and mortality worldwide. Cholesterol gallstone disease is caused by complex genetic and environmental factors and is one of the most prevalent and costly digestive diseases in the USA and Europe. Although sitosterolemia is a rare inherited lipid storage disease, its genetic studies led to identification of the sterol efflux transporters ABCG5/G8 that are located on chromosome 2p21 in humans and chromosome 17 in mice. Human and animal studies have clearly demonstrated that ABCG5/G8 play a critical role in regulating hepatic secretion and intestinal absorption of cholesterol and plant sterols. Sitosterolemia is caused by a mutation in either the ABCG5 or the ABCG8 gene alone, but not in both simultaneously. Polymorphisms in the ABCG5/G8 genes are associated with abnormal plasma cholesterol metabolism and may play a key role in the genetic determination of plasma cholesterol concentrations. Moreover, ABCG5/G8 is a new gallstone gene, LITH9. Gallstone-associated variants in ABCG5/G8 are involved in the pathogenesis of cholesterol gallstones in European, Asian, and South American populations. In this chapter, we summarize the latest advances in the critical role of the sterol efflux transporters ABCG5/G8 in regulating hepatic secretion of biliary cholesterol, intestinal absorption of cholesterol and plant sterols, the classical reverse cholesterol transport, and the newly established transintestinal cholesterol excretion, as well as in the pathogenesis and pathophysiology of ABCG5/G8-related metabolic diseases such as sitosterolemia, cardiovascular disease, and cholesterol gallstone disease.

Bile Acid Physiology

The primary bile acids (BAs) are synthetized from colesterol in the liver, conjugated to glycine or taurine to increase their solubility, secreted into bile, concentrated in the gallbladder during fasting, and expelled in the intestine in response to dietary fat, as well as bio-transformed in the colon to the secondary BAs by the gut microbiota, reabsorbed in the ileum and colon back to the liver, and minimally lost in the feces. BAs in the intestine not only regulate the digestion and absorption of cholesterol, triglycerides, and fat-soluble vitamins, but also play a key role as signaling molecules in modulating epithelial cell proliferation, gene expression, and lipid and glucose metabolism by activating farnesoid X receptor (FXR) and G-protein-coupled bile acid receptor-1 (GPBAR-1, also known as TGR5) in the liver, intestine, muscle and brown adipose tissue. Recent studies have revealed the metabolic pathways of FXR and GPBAR-1 involved in the biosynthesis and enterohepatic circulation of BAs and their functions as signaling molecules on lipid and glucose metabolism.

Cholesterol and Plant Sterol Efflux from Cultured Intestinal Epithelial Cells Is Mediated by ATP-Binding Cassette Transporters

In this study we analyzed functions of ATP-binding cassette (ABC) transporters involved in sterol transport from Caco-2 cells. Treatment with a synthetic liver x receptor ligand elevated both mRNA and protein levels of ABCG5, G8, and ABCA1. The ligand stimulated cholesterol efflux, suggesting that ABC transporters are involved in it. To identify the acceptors of cholesterol, potential molecules such as apolipoprotein A-I, glycocholic acid, phosphatidylcholine, and bile acid micelles were added to the medium. Apo A-I, a known acceptor of cholesterol transported by ABCA1, elevated cholesterol efflux on the basal side, whereas the others raised cholesterol efflux on the apical side. Moreover, bile acid micelles preferentially augmented plant sterol efflux rather than cholesterol. Finally, in HEK293 cells stably expressing ABCG5/G8, bile acid micelle-mediated sterol efflux was significantly accelerated. These results indicate that ABCG5/G8, unlike ABCA1, together with bile acids should participate in sterol efflux on the apical surface of Caco-2 cells.

Comparative proteomic analysis of gallbladder bile proteins related to cholesterol gallstones

Background

Nucleation of cholesterol monohydrate crystals following the aggregation and fusion of cholesterol-enriched vesicles is a critical procedure in the formation of cholesterol gallstone. Biliary proteins play important roles in the process. It is inefficient to screen pro-nucleating or anti-nucleating proteins with routine physiochemical techniques, by which we discovered several pro-nucleating proteins.

Methodology/Principal Findings

Based on comparative proteomic technologies, we investigated the differentially expressed proteins between the cholesterol gallstone and control groups, and between the vesicular phase and micellar phase. There are 401±75 spots detected on the cholesterol gallstone group and 389±94 spots on the control group gels, 120±24 spots detected on vesicular phase and 198±37 on micellar phase gels, and accordingly 22 and 8 differentially expressed proteins were identified successfully, respectively. Three of them, HSA, Profilin and Retinol Binding Protein, were validated by Western blot.

Conclusion/Significance

Some of the identified proteins are in good agreement with proteins reported to be involved in the gallstone formation before. The information from this study might provide some important clues to uncover the key proteins involved in the formation of cholesterol gallstone.

Pathogenesis of gallstones

Cholesterol gallstone formation is a complex process and involves phase separation of cholesterol crystals from supersaturated bile. In most cases, cholesterol hypersecretion is considered the primary event in gallstone formation. The sterol is transported through the hepatocytic canalicular membrane by ABCG5-G8. Expression of this transport protein is regulated by transcription factor Liver X Receptor-alpha, which may be responsible for biliary hypersecretion. Hydrophobic bile salt pool, bile concentration, excess pronucleating mucin, and impaired gallbladder and intestinal motility are secondary phenomena in most cases but nevertheless may contribute to gallstone formation.

Estimation of cholesterol solubilization by a mixed micelle binding model in aqueous tauroursodeoxycholate:lecithin:cholesterol solutions

In order to interpret the clinical efficacy of conjugated ursodeoxycholate (UDC) in cholesterol (Ch) gallstone patients, the Ch solubilization in mixed micelles in 40:40:32 mM tauroursodeoxycholate (TUDC):taurochenodeoxycholate (TCDC):lecithin (L) and 80:32 mM TUDC:L systems was estimated by using a model of Ch binding to mixed micelles. The Ch solubilization limit in mixed TUDC:L micelles was found to be higher than that in mixed TUDC:TCDC:L micelles. In the 80:32 mM TUDC:L system, the dissolution of the Ch pellet decreased after vesicles (liposomes) formed on the surface of the Ch pellet whereas the dissolution of microcrystalline Ch was rapid before and after vesicle formation in the solution, indicating that the total surface area of solid Ch exposed to the solution may be another important factor in inducing the dissolution of Ch gallstones. These phenomena suggest that although vesicles, occasionally formed in the bile of patients under the therapy of conjugated UDC, make a contribution to the solubilization of Ch gallstones, the model of Ch binding to mixed TUDC:L micelles can be used to estimate Ch solubility in TUDC:L system.

The mechanism of ABCG5/ABCG8 in biliary cholesterol secretion in mice

The main player in biliary cholesterol secretion is the heterodimeric transporter complex, ABCG5/ABCG8, the function of which is necessary for the majority of sterols secreted into bile. It is not clear whether the primary step in this process is flopping of cholesterol from the inner to the outer leaflet of the canalicular membrane, with desorption by mixed micelles, or decreasing of the activation energy required for cholesterol desorption from the outer membrane leaflet. In this study, we investigated these mechanisms by infusing Abcg8(+/+), Abcg8(+/-), and Abcg8(-/-) mice with hydrophilic and hydrophobic bile salts. In Abcg8(-/-) mice, this failed to substantially stimulate biliary cholesterol secretion. Infusion of the hydrophobic bile salt taurodeoxycholate also resulted in cholestasis, which was induced in Abcg8(-/-) mice at a much lower infusion rate compared with Abc8(-/-) and Abcg8(+/-) mice, suggesting a reduced cholesterol content in the outer leaflet of the canalicular membrane. Indeed, isolation of canalicular membranes revealed a reduction of 45% in cholesterol content under these conditions in Abcg8(-/-) mice. Our data support the model that ABCG5/ABCG8 primarily play a role in flopping cholesterol (and sterols) from the inner leaflet to the outer leaflet of the canalicular membrane.

Gallbladder Na+/H+ exchange activity is up-regulated prior to cholesterol crystal formation

BACKGROUND

Gallbladder Na+ and H2O absorption are increased prior to gallstone formation and may promote cholesterol nucleation. Na+/H+ exchange (NHE) isoforms NHE2 and NHE3 are involved in gallbladder Na+ transport in prairie dogs. We examined whether increased gallbladder Na+ absorption observed during early gallstone formation is the result of NHE up-regulation.

MATERIALS AND METHODS

Native gallbladder and primary cultures of gallbladder epithelial cells (GBECs) harvested from prairie dogs fed nonlithogenic (CON) or 1.2% cholesterol diet for varying lengths of time to induce cholesterol-saturated bile (PreCRYS), cholesterol crystals (CRYS), or gallstones (GS) were used. NHE activity was assessed by measuring dimethylamiloride-inhibitable 22Na+ uptake under H+ gradient in primary GBECs. HOE-694 was used to determine NHE2 and NHE3 contributions. NHE protein and mRNA expression were examined by Western and Northern blots, respectively.

RESULTS

Gallbladder total NHE activity was 25.1 +/- 1.3 nmol mg protein(-1) min(-1) in the control and increased during gallstone formation peaking at the PreCRYS stage (98.4 +/- 3.9 nmol mg protein(-1) min(-1)). There was a shift in NHE activity from NHE2 to NHE3 as the animals progressed from no stones through the PreCRYS and CRYS stages to gallstones. The increase in NHE activity was partly caused by an increased Vmax without any change in K(Na)m. Both NHE2 and NHE3 protein increased moderately during the PreCRYS stage without increases in mRNA expression.

CONCLUSIONS

Increased gallbladder Na+ absorption observed prior to crystal formation is in part caused by an increase NHE activity which is not fully accounted for by an increase in NHE proteins and mRNA levels but may be explained by enhanced localization in the membranes and/or altered regulation of NHE.

Solubility of cholesterol in the crystal-free gallbladder bile of gallstone patients

Supersaturation of bile with cholesterol is generally considered the driving force of cholesterol precipitation. However, in most investigations the amount of cholesterol crystals is included in the calculation of the cholesterol saturation index (CSI). We therefore studied the solubility of cholesterol in crystal-free gallbladder bile from gallstone patients. Our results demonstrate significantly ( P <.05) higher CSIs (1.4 +/- 0.5 and 1.4 +/- 0.4 vs 1.1 +/- 0.4, mean +/- SD) in crystal-free gallbladder bile from 66 patients with cholesterol stones and 21 patients with mixed stones compared with those in 30 patients with pigment stones and a significant difference ( P <.001) in the amount of cholesterol in vesicles (19.2% +/- 13.7% and 14.3% +/- 11.6 % vs 4.2% +/- 5.9%) and of the crystal-observation time (COT; 1-21 days, median 2 days and 1-21 days, median 3 days, vs 3-21 days, median 21 days). We detected a positive correlation ( r =.24, P <.01) between the percentage of cholesterol in vesicles and the CSI and a negative correlation between COT and CSI ( r = -.23, P <.02 ) and COT and the percentage of cholesterol in vesicles ( r = -.52, P <.001 ). However, in 14 of 30 gallbladder-bile specimens from patients with pigment stones but in just 5 of 21 specimens from patients with mixed stones patients and 12 of 66 specimens from patients with cholesterol stones, the distribution of cholesterol in different phases (mixed micelles, vesicles, and crystals) was within the limits of solubility determined in previous studies of model bile. Therefore, in addition to the relative composition of biliary lipids, nonlipid components exert considerable influence on the solubility of cholesterol in the gallbladder bile of gallstone patients.

Interaction of apolipoprotein A-I with lecithin-cholesterol vesicles in the presence of phospholipase C

Here we study the anti-nucleating mechanism of apolipoprotein A-I (apo A-I) on model biliary vesicles in the presence of phospholipase C (PLC) utilizing dynamic light scattering (DLS), steady-state fluorescence spectroscopy, cryogenic transmission electron microscopy (cryo-TEM), and UV/Vis spectroscopy. PLC induces aggregation of cholesterol-free lecithin vesicles from an initial, average size of 100 nm to a maximal size of 600 nm. The presence of apo A-I likely inhibits vesicle aggregation by shielding the PLC-generated hydrophobic moieties, which results in vesicles of an average size of 200 nm. A similar phenomenon is observed in cholesterol-enriched lecithin vesicles. Whereas PLC alone produces aggregates of 300 nm, no aggregation is observed when apo A-I is present along with PLC. However, the ability of apo A-I to inhibit aggregation is temporary, and after 8 h, a broad particle size distribution with sizes as high as 800 nm is observed. Apo A-I possibly induces the formation of small apo A-I/lecithin/cholesterol complexes of about 5-20 nm similar to the discoidal pre-HDL complexes found in blood when it can no longer effectively shield all the DAG molecules. Concomitant with formation of complexes, DAG molecules coalesce into large oil droplets, which account for the large particles observed by light scattering. Thus, apo A-I acts as an anti-nucleating agent by two mechanisms, anti-aggregation and microstructural transition. The mode of protection is dependent on the cholesterol content and the relative amounts of DAG and apo A-I present. This study supports the possibility of apo A-I solubilizing lipids in bile in a similar fashion as it does in blood and also delineates the mechanism of formation of the complexes.

Biliary lipid composition in cholesterol microlithiasis

BACKGROUND

Little information is available on the pathogenesis of cholesterol microlithiasis, and it is not clear if biliary lipid composition in these patients is similar to changes seen in cholesterol gall stone patients.

AIMS

To measure biliary lipid composition in patients with cholesterol microlithiasis.

PATIENTS

Eleven patients with cholesterol microlithiasis, 20 cholesterol gall stone patients, and 17 healthy controls.

METHODS

Duodenal bile was collected in the fasting state during ceruletide infusion. Biliary cholesterol, phospholipids, and total bile acids were analysed by enzymatic assays, and conjugated bile acids by high pressure liquid chromatography.

RESULTS

Patients with microlithiasis had a cholesterol saturation index significantly higher than controls (mean value 1.30 (95% confidence interval 1.05-1.54) v 0.90 (0.72-1.08)) but similar to gall stone patients (1.51 (1.40-1.63)). This was due to a significant decrease in per cent phospholipid (10.0% (7.1-12.8)) compared with controls (21.4% (18.1-24.6)) and gall stone patients (24.9% (20.5-29.3)). Per cent cholesterol was similar in patients with microlithiasis and controls (5.3% (4.5-6.1) and 5.6 % (4.3-6.8), respectively) but was significantly increased in gall stone patients (10.9% (9.3-12.4)). Bile acid composition in patients with microlithiasis was similar to controls whereas in gall stone patients deoxycholic acid was significantly increased: 27.3% (24.8-29.7) v 19.0% (15.7-22.2) in controls and 20.6% (14.9-26.2) in patients with microlithiasis.

CONCLUSION

Patients with cholesterol microlithiasis have biliary cholesterol supersaturation, similarly to cholesterol gall stone patients. Whereas in the latter this is due to increased per cent cholesterol, in patients with microlithiasis this is caused by phospholipid deficiency, with normal per cent cholesterol and normal biliary bile acid composition.

Lipid solubilization in human gallbladder versus hepatic biles

BACKGROUND/AIMS

Cholesterol crystallizes more rapidly in gallbladder than in hepatic biles, supposedly due to formation of cholesterol-supersaturated vesicles in concentrated gallbladder biles because of preferential micellization of phospholipids compared to cholesterol. We therefore aimed to compare lipid solubilization in hepatic and gallbladder biles.

METHODS

Mixed micellar and vesicular phases were separated from hepatic and associated gallbladder biles of seven cholesterol gallstone patients by using state-of-the-art gel filtration with bile salts at intermixed micellar/intervesicular compositions and concentrations in the eluant.

RESULTS

Vesicles were found in 6 out of 7 hepatic biles, but only in 2 of the corresponding gallbladder biles. Both percentage (7.8+/-5.1 vs. 36.3+/-7.6%; p = 0.01) and amount (0.9+/-0.2 vs. 1.7+/-0.3 mM; p = 0.06) of vesicular cholesterol were lower in gallbladder biles. Similar results were found for vesicular phospholipids (1.3+/-0.8 vs. 11.6+/-6.0%; p = 0.05; and 0.3+/-0.1 vs. 1.1+/-0.5 mM; p = 0.07). The vesicular cholesterol/ phospholipid ratio was 1.7+/-0.5 in hepatic bile but 4.3 and 1.8 in the 2 gallbladder biles which contained vesicles. Mixed micelles in gallbladder biles had a higher cholesterol saturation index than mixed micelles in hepatic biles (1.43+/-0.11 vs. 1.15+/-0.07; p = 0.02).

CONCLUSIONS

Concentration of bile in the gallbladder leads to decreased vesicular lipid contents. The finding of supersaturated mixed micelles in the absence of vesicles in a significant number of patients points to the possibility of non-vesicular modes of crystallization.

Physical and metabolic factors in gallstone pathogenesis

Gallstones form when the tenuous balance of solubility of biliary lipids tips in favor of precipitation of cholesterol, unconjugated bilirubin, or bacterial degradation products of biliary lipids. For cholesterol gallstones, metabolic alterations in hepatic cholesterol secretion combine with changes in gallbladder motility and intestinal bacterial degradation of bile salts to destabilize cholesterol carriers in bile and produce cholesterol crystals. For black pigment gallstones, changes in heme metabolism or bilirubin absorption lead to increased bilirubin concentrations and precipitation of calcium bilirubinate. In contrast, mechanical obstruction of the biliary tract is the major factor leading to bacterial degradation and precipitation of biliary lipids in brown pigment stones. Further understanding of the physical and metabolic factors of cholesterol and black pigment formation is likely to provide interventions to interrupt the earliest stages of gallstone formation.

Accurate separation of biliary lipid aggregates requires the correct intermixed micellar/intervesicular bile salt concentration

The intermixed micellar/intervesicular bile salt (BS) concentration (IMC), composed of BS monomers and simple micelles, is in dynamic equilibrium with mixed micelles and vesicles. Accurate separation of biliary lipid aggregates is believed to depend on accurately measuring the IMC. Using centrifugal ultrafiltration, we measured the IMC of cholesterol-supersaturated model biles that were physiologically composed. Gel chromatography was performed using eluants containing the following: 1) the IMC; 2) the same BS composition as the IMC but higher or lower BS concentrations; 3) the same BS concentration as the IMC, but with more hydrophilic or hydrophobic BS; and 4) 10 mmol/L cholate. Compared with an eluant containing the same BS composition as the IMC, an eluant containing the same relative BS composition but 75% of the IMC increased the proportion of cholesterol in vesicles and decreased the vesicular cholesterol/egg yolk phosphatidylcholine (EYPC) ratio. In contrast, an eluant containing 150% of the IMC entirely transformed vesicles to micelles. Eluants containing slightly more hydrophobic or more hydrophilic BS eliminated or increased vesicular cholesterol content, respectively. An eluant of 10 mmol/L cholate overestimated vesicular cholesterol and in concentrated biles reproducibly produced an incompletely separated intermediate peak, possibly because of re-equilibration between mixed micelles and vesicles. Further, in concentrated biles, fractions eluting at volumes corresponding to mixed micelles were visibly turbid, irrespective of the eluant used. The correct IMC allows accurate separation of biliary lipid aggregates, but differences in BS concentration or composition substantially alter the vesicular percentage of cholesterol as well as the cholesterol/EYPC ratio. Elution with 10 mmol/L cholate may introduce artifactual gel-filtration peaks and inadequate separation of particles with widely differing molecular weights, both of which have confused previous analyses of biliary lipid aggregates.

Protein lipid interaction in bile: effects of biliary proteins on the stability of cholesterol-lecithin vesicles

The nucleation of cholesterol crystals is an obligatory precursor to cholesterol gallstone formation. Nucleation, in turn, is believed to be preceded by aggregation and fusion of cholesterol-rich vesicles. We have investigated the effects of two putative pro-nucleating proteins, a concanavalin A-binding protein fraction and a calcium-binding protein, on the stability of sonicated small unilamellar cholesterol-lecithin vesicles. Vesicle aggregation is followed by monitoring absorbance, and upon addition of the concanavalin A-binding protein fraction the absorbance of a vesicle dispersion increases continuously with time. Vesicle fusion is probed by a fluorescence contents-mixing assay. Vesicles apparently fuse slowly after the addition of the concanavalin A-binding protein, although inner filter effects confound the quantitative measurement of fusion rates. The rates of change of absorbance and fluorescence increase with the concentration of the protein, and the second-order dimerization rate constant increases with both the protein concentration and the cholesterol content of the vesicles. On the other hand, the calcium-binding protein has no effect on the stability of the vesicle dispersion. This protein may therefore affect cholesterol crystal formation not by promoting the nucleation process, but by enhancing crystal growth and packaging. Our results demonstrate that biliary proteins can destabilize lipid vesicles and that different proteins play different roles in the mechanism of cholesterol gallstone formation.

Imaging supramolecular aggregates in bile models and human bile

Investigation of cholesterol crystallization is essential for the understanding of gallstone formation. Previous work has revealed a variety of aggregates of different sizes and shapes prior to the appearance of "classical" plate-like cholesterol monohydrate crystals both in native biles and model systems. In this article, we review existing data based on various microscopic techniques and present data on microstructural pathways leading to cholesterol crystal formation in two different bile models and in native bile. In continuation of our recent investigation of microstructures in nucleating human bile, we now present data suggesting that polymorphism is not limited to complex native bile, but also appears in two, simplified model systems. These studies employed cryo-transmission electron microscopy (cryo-TEM) and video-enhanced light microscopy, using Nomarski optics (VELM). Only the combined use of these two complementary, non-perturbing direct methods can cover the whole range of microstructures ranging from a few nanometers to several microns. Concentrated isotropic solutions of bile models, composed of cholesterol, lecithin and taurocholate, were diluted to induce cholesterol supersaturation and start an evolution of microstructures, leading to cholesterol crystallization. Initially, small spheroidal micelles were observed by cryo-TEM. Subsequently, uni-, oligo- and multilamellar vesicles, compatible with structures seen at the same time by VELM, appeared in coexistence with micelles. Thereafter, during a dynamic phase of cholesterol crystallization, filaments, tubular and helical microstructures, as well as classical plate-like cholesterol monohydrate crystals were noted by light microscopy. Eventually, large plate-like crystals were observed by VELM, while cryo-TEM revealed only small spheroidal micelles. The crystallization process in native human bile during ex vivo incubation was found to bear close resemblance to the findings in the model systems, further supporting the applicability of these systems to the exploration of microstructural aspects of nucleating human bile.

Two-dimensional electrophoretic analysis of vesicular and micellar proteins of gallbladder bile

Proteins associated with lipid vesicles or mixed micelles of human gallbladder bile were separated by Sepharose-2B gel filtration chromatography followed by protein concentration and delipidation. After two-dimensional polyacrylamide gel electrophoresis and silver staining up to 59 and 471 polypeptide spots were counted in vesicular and micellar fractions, respectively. As major components the plasma proteins transferrin, albumin, alpha-fibrinogen, beta-fibrinogen, gamma-immunoglobulin G, immunoglobulin light chains, alpha-1 antitrypsin and haptoglobin alpha-2 chain were identified in the lipid vesicles by comparison with human protein reference maps. However, most biliary proteins including the anionic polypeptide fraction are associated with mixed micelles. The pathophysiological significance of these proteins associated with lipids needs to be investigated further.

Structural mechanisms of bile salt-induced growth of small unilamellar cholesterol-lecithin vesicles

The liver secretes cholesterol and lecithin in the form of mixed vesicles during the formation of bile. When exposed to bile salts, these metastable vesicles undergo various structural rearrangements. We have examined the effects of three different bile salts, taurocholate (TC), tauroursodeoxycholate (TUDC), and taurodeoxycholate (TDC), on the stability of sonicated lecithin vesicles containing various amounts of cholesterol. Vesicle growth was probed by turbidity measurements, quasi-elastic light scattering, and a resonance energy transfer lipid-mixing assay. Leakage of internal contents was monitored by encapsulation of fluorescence probes in vesicles. At low bile salt-to-lecithin ratios (TC/L or TUDC/L < 1), pure lecithin vesicles do not grow, but exhibit slow intervesicular mixing of lipids as well as gradual leakage. At high BS/L (TC/L or TUDC/L > 5), pure lecithin vesicles are solubilized into mixed micelles with a concomitant decrease in the overall particle size. In this regime, extensive leakage and lipid mixing occur instantaneously after exposure to bile salt. At intermediate BS/L (1 < TC/L or TUDC/L < 5), vesicles grow with time, and the rates of both leakage and lipid mixing are rapid. The data suggest that vesicles grow by the transfer of lecithin and cholesterol via diffusion in the aqueous medium. The addition of cholesterol to lecithin vesicles reduces leakage dramatically and increases the amount of BS required for complete solubilization of vesicles. The more hydrophobic TDC induces vesicle growth at a lower BS/L than does TC or TUDC. These results demonstrate the physiologic forms of lipid microstructures during bile formation and explain how the hydrophilic-hydrophobic balance of BS mixtures may profoundly affect the early stages of CH gallstone formation.

Monitoring cholesterol crystallization from lithogenic model bile by time-lapse density gradient ultracentrifugation

BACKGROUND/AIMS

Cholesterol crystallization in a dilute, bile salt-rich model bile is a multiphase process in which early filamentous crystals gradually transform to classical cholesterol monohydrate plates. The pertinence of similar transformations in more complex model systems or native bile is, however, unclear. The aim of the present study was to characterize and monitor cholesterol crystallization in a model bile of physiological relevance.

METHODS

A supersaturated model bile was prepared with a lipid composition (18 mM cholesterol, 37 mM lecithin, 120 mM taurocholate) that was derived from analyzing 10 gallbladder biles from cholesterol gallstone patients. Cholesterol crystallization was followed by light and electron microscopy, and sequential density gradient analysis of cholesterol-containing precipitates.

RESULTS

During cholesterol crystallization a reproducible sequence of events was recorded. First (T<18 h), cholesterol-rich vesicular and multilamellar structures (density 1.005-1.015 g/ml) were observed. Later, (T>60 h) filamentous, helical, tubular (density 1.015-1.04 g/ml) and plate-like (density 1.04-1.06 g/ml) cholesterol crystals appeared. The concentration of crystals increased gradually, while bilayer structures became desaturated with cholesterol and disappeared, and early crystal forms were replaced by plates. Eventually (T>25 days) only classical plate-like cholesterol monohydrate crystals were present. Exposure of cholesterol-containing precipitates to micellar (100 mM) deoxycholate dissolved the bilayer structures but not the crystals.

CONCLUSIONS

These data demonstrate that cholesterol crystallization in a physiologically relevant model bile is a multiphase process consisting of a sequence of transitions from vesicular and multilamellar structures to early crystal forms and to classical plate-like cholesterol monohydrate crystals. These transitions are associated with increasing density and decreasing phospholipid content of cholesterol precipitates. We suggest that time-lapse density gradient ultracentrifugation is a useful method for investigating and quantitating the process of cholesterol crystallization and factors that influence this process in bile.

Quantifying vesicle/mixed micelle partitioning of phosphatidylcholine in model bile by using radiolabeled phosphatidylcholine species

Phospholipid in vesicles and mixed micelles of (model) bile has been traced or quantitated (or both) by adding radioactively labeled phosphatidylcholine species. The question is whether these labeled species mix homogeneously with the phosphatidylcholine species mixture present, such that the label distribution reflects the already established mass partitioning of species. In this study, model bile containing egg yolk phosphatidylcholine was incubated with radioactive phosphatidylcholine species. Vesicle and mixed micelle fractions were separated by gel filtration. Radiochemical analysis of the species distribution confirmed chemical analysis: 1,2-di(14C)palmitoyl-phosphatidylcholine was enriched in the vesicles, the 1-palmitoyl-2-(14C)oleoyl species evenly distributed, and the 1-palmitoyl-2-(14C)linoleoyl species more expressed in mixed micelles. This indicates that the distribution of an added radioactive phosphatidylcholine species represents the vesicle/mixed micelle distribution of that particular phosphatidylcholine species. Consequently, the label distribution of a particular added radioactive phosphatidylcholine species can be used to calculate the vesicle/mixed micelle partitioning of total phosphatidylcholine only after it has been established that the radioactive species reaches the same partitioning as total phosphatidylcholine.

An improved ultracentrifugation method for the separation of cholesterol carriers in bile

Vesicles and micelles, the major carriers of cholesterol in bile, play a role in the formation of cholesterol gallstones. A simple and rapid ultracentrifugation method was developed to isolate these biliary cholesterol carriers when only microliter amounts of bile were available. The proposed method employs a 46 to 0% sucrose density gradient, a NVT90 near-vertical rotor, and a centrifugation time of one hour. As little as 25 microL of bile can be used with no disruption of the carriers. The method was validated by comparison with gel filtration column chromatography using 6 mM taurocholate in the elution buffer. The sucrose linear density gradient ultracentrifugation procedure described here is simple, fast, and compares favorably with the gel filtration chromatography method for the separation of cholesterol carriers from bile.

Lithogenicity of human bile is reduced by freezing and thawing

We examined the effects of freezing and thawing upon the nucleation time and the distribution of cholesterol between micelles and vesicles in 9 human gallbladder and 7 hepatic biles. The nucleation time was significantly longer after freezing when compared to fresh samples (22.4 +/- 2.6 vs. 7.4 +/- 1.9 days, respectively). Concomitantly, a substantial shift of cholesterol from vesicles to micelles was noted, with the proportion of vesicular cholesterol decreasing from 26.5% +/- 6.0% in fresh biles to 8.6% +/- 2.3% after freezing. These effects were observed in all types of human biles, regardless of origin, cholesterol saturation or initial presence of cholesterol crystals, and were most notable after the first week of freezing. The decrease in vesicular cholesterol in all biles and the increase in nucleation time of gallbladder biles correlated with the time the samples had been in a frozen state. It is concluded that the lithogenic properties of human bile are not maintained during storage at -20 degrees C. Freezing results in a shift of cholesterol from vesicles to micelles and reduces the tendency of cholesterol to crystallize from bile samples.

The effects of dietary phospholipids enriched with phosphatidylethanolamine on bile and red cell membrane lipids in humans

The role of phospholipids in biliary cholesterol solubilization and crystallization has only recently begun to be appreciated. Phospholipid vesicles are believed to be the metastable carrier from which cholesterol nucleates. Cholesterol crystallization is influenced by the phospholipid species in bile. Feeding rats and hamsters with diets enriched in phospholipids or their precursors, especially ethanolamine, resulted in reduced cholesterol saturation of bile. Although whole phospholipids are normal dietary constituents, the effects and safety of phospholipid components have not been tested in humans. In the present study, we have evaluated the effects of a dietary phospholipid mixture, enriched with phosphatidylethanolamine, on human bile and red blood cell membrane lipid composition. Five ambulatory volunteers having a chronic indwelling T-tube, with an intact enterohepatic circulation, were investigated. Thirty-six grams of phospholipids (54% phosphatidylethanolamine, 54% linoleyl acyl chains) were added to their daily diet for fourteen days. Biliary nucleation time, cholesterol carriers, as well as plasma, red blood cell membrane, and bile lipid compositions, were monitored. Following phospholipid supplementation, the proportion of linoleyl chains (18:2) in biliary phospholipids increased significantly from 31.1 +/- 1.2 to 37.7 +/- 5.3%, while that of oleyl chains (18:1) decreased from 11.4 +/- 1.6 to 9.6 +/- 1.1%. These changes were accompanied by an increase of linoleate and its metabolite, arachidonate, in red cell membranes. Phospholipid feeding did not cause any side effects, and no significant changes in biliary nucleation time, cholesterol, phospholipid, or bile salt concentrations, or in the distribution of cholesterol within micelles or vesicles. We conclude that phospholipid feeding is safe, and can be effective as a vehicle for lecithin fatty acyl chain modulation of bile and lipid membranes. These findings may provide a basis for a controlled modulation of biliary phospholipids to increase cholesterol solubility in bile.

In vitro determination by 1H-NMR studies that bile with shorter nucleation times contain cholesterol-enriched vesicles

Although biliary vesicles are considered to be the primary source of cholesterol found in cholesterol gallstones, difficulties in quantitatively separating the different cholesterol transport modes in bile still remain. Proton nuclear magnetic resonance spectroscopy (1H-NMR) offers an alternative approach. Investigations were carried out on both model biles and human gallbladder bile samples: (i) to follow the effect of increasing sodium glycocholate concentrations on the 1H-NMR spectra of arachidonic acid rich-phospholipid, and cholesterol-lecithin vesicles, (ii) to compare the concentrations of total phospholipids in bile determined enzymatically with those obtained by integration of the phospholipid choline head group resonance peak, and (iii) to examine the relationship between biliary cholesterol nucleation time (NT) and the areas of the biliary lipid 1H-NMR peaks. It was found that the molecular motions of vesicle phospholipid, as determined by 1H-NMR, were restricted by saturation with cholesterol. In bile from patients with cholesterol gallstones, the reduced NMR fluidity of the phospholipid choline-head group indicated that the proportion of cholesterol-phospholipid vesicles containing more than 50% cholesterol, on a molar basis, was increased. The ratios of the N+(CH3)3 and = CH proton resonance peaks showed no overlap between samples with cholesterol gallstones and shorter NT and those with either no gallstones or pigment stones and longer NT. 1H-NMR spectroscopy indicates in a non-invasive manner those biles which are prone to cholesterol crystal formation.

Lamellar bodies coexist with vesicles and micelles in human gallbladder bile. Ursodeoxycholic acid prevents cholesterol crystal nucleation by increasing biliary lamellae

The aggregative forms of lipids in human gallbladder bile and their relation to cholesterol crystallization are controversial. Using combined chemical, gel-chromatographic, optical/electron microscopic and quasielastic light-scattering methods, we investigated this issue in native gallbladder bile obtained from nine untreated cholesterol gallstone patients and eight cholesterol gallstone patients treated for 1 week with 600 mg/day of ursodeoxycholic acid. Bile obtained at cholecystectomy was ultracentrifuged for 2 h at 150,000 g to obtain isotropic samples. The conventional cholesterol crystal observation time was 3.1 +/- 4.1 (SD) days in controls and 19.0 +/- 1.9 days in the ursodeoxycholic acid-treated group (p < 0.001). Bile was analyzed by high-resolution gel-chromatography using 7 mM sodium taurocholate in the elution buffer. Biliary lipids eluted in four chromatographic zones: zone #I, corresponding to the column void volume, contained only minimal amounts of lipids; zone #II (apparent m.w. 100-220 kDa) comprised 29.1 +/- 12.4% of biliary cholesterol in the untreated group and 8.3 +/- 4.3% in the ursodeoxycholic acid-group (p < 0.001). At negative staining electron microscopy, this region was composed of roundish vesicles ranging from 7 to 20 nm in diameter. Zone #III (apparent m.w. 50-100 kDa) carried 59.1 +/- 2.1% of cholesterol in untreated patients and 81.2 +/- 9.5% in ursodeoxycholic acid-rich biles, respectively (p < 0.001). At negative staining electron microscopy, this region was composed of lamellar stacks of variable length, usually with 5 nm interspaces and up to 30 nm in width. In ursodeoxycholic acid-rich biles, lamellae often appeared in the form of concentric fingerprint-like images. Quasielastic light-scattering measurements in this region were compatible with the size estimates obtained at electron microscopy. Zone #IV (apparent m.w. 6-50 kDa) carried 11.8 +/- 9.4% and 11.6 +/- 9.0% of cholesterol, respectively (not significant). Since this region comprised a considerable fraction of endogenous bile salts and had no distinct morphological structures, it was interpreted as mixed micelles. The cholesterol crystal observation time showed a significant inverse correlation (r = -0.85, p < 0.001) with percent cholesterol carried by vesicles (zone #II) and a direct correlation (r = 0.86, p < 0.001) with percent cholesterol carried by lamellar bodies (zone #III). Vesicles and lamellae identical to those observed in isolated gel-chromatographic fractions were observed also on direct electron microscopic examination of unfractionated isotropic native biles. Similar findings were observed also in matched model biles.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental evaluation of a model for predicting micellar composition and concentration of monomeric species in bile salt binary mixtures

The critical micellar concentration (cmc) values of some mixed systems containing two bile salts were determined by a maximum pressure bubble method and compared with those derived from a theoretical model developed for nonionic surfactants to assess the applicability of this model to such systems. Some assumptions on which the presumed validity of this model was based are discussed. The following binary mixtures were investigated: sodium chenodeoxycholate with cholate, ursocholate and ursodeoxycholate, either unconjugated or conjugated with taurine and glycine at different mole fractions (0, 0.25, 0.5, 0.75, 1) in 0.15 M NaCl. For these mixtures, experimentally determined data were in good agreement with values predicted by the theoretical model: both the cmc and the surface tension at this concentration of the mixtures were intermediate between those of the two pure bile salts; also, as the total bile salt concentration increased, the mixed micelles became enriched with the bile salt having the highest cmc, whereas the total monomer activity, determined by a potentiometric method employing a bile salt-selective electrode, increased only slightly. To test this model in an in vitro system, surface tension was also measured in ox bile samples that were enriched by 50% with sodium ursodeoxycholate, chenodeoxycholate, or their taurine amidates. The cmc and the surface tension at this concentration of the artificial bile increased when enriched with a bile salt with a cmc higher than that of endogenous salts (e.g. ursodeoxycholate versus taurocholate), whereas the reverse occurred for mixtures enriched with a bile salt with a lower cmc, such as chenodeoxycholate.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary fat alters the distribution of cholesterol between vesicles and micelles in hamster bile

The type of dietary fat strongly affects the incidence of gallstones in the hamster model of cholesterol cholelithiasis. The present study was designed to determine whether dietary fats could affect gallstone formation by altering the microstructure (vesicular/micellar ratio) of cholesterol in bile. Golden Syrian hamsters from Sasco (Omaha, NE) or Charles River (Wilmington, MA) were fed nutritionally adequate semipurified diets to which were added: (i) 4.0% butterfat without added cholesterol; (ii) 1.2% palmitic acid plus 0.3% cholesterol; or (iii) 4.0% safflower oil plus 0.3% cholesterol. Gallstone incidence and the percentage of cholesterol in vesicles and micelles were determined after two- or six-week feeding periods. Three out of ten Sasco hamsters fed the 1.2% palmitic acid diet for two weeks had cholesterol stones, while none of the eight Charles River animals had stones. In the Sasco hamsters, a significant proportion of the biliary cholesterol was found in void volume vesicles (28.8%) and small vesicles (17.1%); Charles River hamsters had negligible proportions (1.1%) of cholesterol in void volume vesicles and 15.4% in small vesicles. Cholesterol gallstones were most abundant in Sasco hamsters fed 1.2% palmitic acid for six weeks (nine out of ten animals); the mean cholesterol saturation index of the bile was 1.27. A significant proportion of the biliary cholesterol was eluted in the void volume vesicles (21.4%) and in small vesicles (15.0%). Five of the eight identically treated Charles River hamsters had cholesterol stones; the cholesterol saturation index averaged 1.36, and the biliary cholesterol was present in void volume vesicles (31.3%) and small vesicles (14.3%).(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of apolipoprotein A1 in cholesterol gallstones and gallbladder bile of patients with gallstones

Biliary apolipoprotein A1 in bile inhibits the nucleation of cholesterol crystals from bile super-saturated with cholesterol. In the present study, using an enzyme-linked immunosorbent assay of apolipoprotein A1, we determined the content of apolipoprotein A1 in cholesterol gallstones and samples of gallbladder bile collected simultaneously from 23 patients during cholecystectomy. Protein content in cholesterol gallstones ranged from 50 to 5700 micrograms/g, with median, quartile, and three quartile values being 250, 111, and 740; apolipoprotein A1 content ranged from 9 to 9000 ng/g (200, 41, 647). The gallbladder bile samples contained protein at concentrations of 0.4-9.0 mg/ml (2.0, 1.1, 3.2), while apolipoprotein A1 was present at concentrations of 2.0-136.0 micrograms/ml (30.0, 10.0, 90.0). A notable finding was that the A1/total protein (TP) values for gallbladder bile, which ranged from 0.13% to 6.80% (1.62, 0.89, 3.34), were several times higher than those determined for gallstone samples, which ranged from 0.01% to 1.2%, 2% (0.06, 0.02, 0.25). The results of sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the protein profile in cholesterol gallstones was similar to that in gallbladder bile. It was concluded that: (1) the protein contained in gallstones may originate from bile, (2) the content of apolipoprotein A1 in cholesterol gallstones is only a trace amount, compared with that in gallbladder bile, and (3) biliary apolipoprotein A1 may be retained in a soluble phase in gallbladder bile, with minimal precipitation onto the surfaces of gallstones.

Distribution of cholesterol among its carriers in the bile of male and female hamsters

The distribution of cholesterol among its carriers was studied in the bile of male and female hamsters. Sasco hamsters (Sasco Inc., Omaha, NE) were fed a semipurified diet with 0.0% cholesterol and 4% butterfat (group 1, males; group 4, females); a semipurified diet with 0.3% cholesterol and 1.2% palmitic acid (group 2, males; group 5, females); and a semipurified diet with 0.3% cholesterol and 4% safflower oil (group 3, males; group 6, females). At the end of six weeks, gallstones were found only in male hamsters receiving both cholesterol and dietary fat (fatty acid) (incidence of cholesterol stones: 90% in group 2; 22% in group 3). The biliary cholesterol carriers were separated and isolated from the bile of the hamsters by gel filtration chromatography, using the method of Pattinson [Pattinson, N.R., Willis, K.E., and Frampton, C.M. (1991) J. Lipid Res. 32, 205-214]. In those male hamsters that formed cholesterol gallstones, significant amounts of cholesterol were present in the void volume which contained large cholesterol phospholipid vesicles (void volume vesicles) (23% in group 2 and 15% in group 3). Smaller cholesterol/phospholipid vesicles were eluted next (fractions 30-45) and contained 15% of biliary cholesterol in group 2 and 21% in group 3. The remainder of the cholesterol was associated with mixed cholesterol/phospholipid/bile salt micelles. The cholesterol/phospholipid ratio was larger in both the void volume vesicles and small vesicles (2.40 and 1.48 in group 2; 2.56 and 1.33 in group 3, respectively) compared to the micelles (about 0.3 in groups 2 and 3).(ABSTRACT TRUNCATED AT 250 WORDS)