Degree of fatty acyl chain unsaturation in biliary lecithin dictates cholesterol nucleation and crystal growth

Paradoxes: Cholesterol and Hypoxia in Preeclampsia

Preeclampsia, a hypertensive disease of pregnancy of unknown etiology, is intensely studied as a model of cardiovascular disease (CVD) not only due to multiple shared pathologic elements but also because changes that develop over decades in CVD appear and resolve within days in preeclampsia. Those affected by preeclampsia and their offspring experience increased lifetime risks of CVD. At the systemic level, preeclampsia is characterized by increased cellular, membrane, and blood levels of cholesterol; however, cholesterol-dependent signaling, such as canonical Wnt/βcatenin, Hedgehog, and endothelial nitric oxide synthase, is downregulated indicating a cholesterol deficit with the upregulation of cholesterol synthesis and efflux. Hypoxia-related signaling in preeclampsia also appears to be paradoxical with increased Hypoxia-Inducible Factors in the placenta but measurably increased oxygen in maternal blood in placental villous spaces. This review addresses the molecular mechanisms by which excessive systemic cholesterol and deficient cholesterol-dependent signaling may arise from the effects of dietary lipid variance and environmental membrane modifiers causing the cellular hypoxia that characterizes preeclampsia.

Nutritional factors (nutritional aspects) in biliary disorders: bile acid and lipid metabolism in gallstone diseases and pancreaticobiliary maljunction

Nutritional factors play a key role in the pathogenesis of biliary diseases such as gallstones and pancreaticobiliary maljunction. Gallstones are primarily classified into cholesterol stone and pigment stone according to the major composition. Cholesterol gallstone formation is very likely based upon supersaturated bile formation, and pigment stones are formed in bile rich in bilirubin. Thus, defects of hepatic metabolism of lipids and organic anions lead to biliary stones. Here, the recent understanding of cholesterol gallstone pathogenesis is elaborated. On the other hand, there is another important link of biliary lipid degradation to serious biliary disease, namely pancreaticobiliary maljunction. Lysophosphatidylcholine (lysoPC), a derivative of phosphatidylcholine hydrolysis by phospholipase A2, is a highly abundant bioactive lipid mediator present in circulation as well as in bile. Increases in bile of lysoPC and phospholipase A2 have been reported in pancreaticobiliary maljunction and considered to be the major risk factor for biliary tract cancers. Further, oxidized fatty acids have been established as a potent ligand for G2A, a member of G protein-coupled receptor family that mediates a diverse array of biological processes including cell growth and apoptosis. Thus, both of lysoPC and free fatty acids are supposed to play an important role through G2A in biliary inflammation and carcinogenesis of pancreaticobiliary maljunction. Taken together, nutritional factors, especially lipid compounds, are seemingly crucial in the pathogenesis of biliary diseases, and such a causal relationship is reviewed by mainly authors' previous publications.

Obese mice incur greater myofiber membrane disruption in response to mechanical load compared with lean mice

OBJECTIVE

Obesity is associated with modified transmembrane signaling events in skeletal muscle, such as insulin signaling and glucose transport. The underlying cause of these obesity-related effects on transmembrane signaling is still unknown. In general, the function of membrane proteins responsible for transmembrane signaling is modulated by the biochemical makeup of the membrane, such as lipid composition, in which they are embedded. Any obesity-related alterations in membrane composition would also be predicted to modify membrane biomechanical properties and membrane susceptibility to mechanical load-induced damage. The primary objective of this study was to investigate whether obesity influences myofiber membrane susceptibility to mechanical damage in skeletal muscle.

DESIGN AND METHODS

Myofiber membrane damage was compared between 12-week-old obese, hypercholesterolemic (B6.V Lep(ob) /J) and isogenic, normocholesterolemic control (C57BL6/J) male mice following either normal cage activity or strenuous eccentric exercise (downhill running). Myofiber membrane damage was quantified in perfusion-fixed frozen sections of the gastrocnemius muscle via sarcoplasmic concentration of either albumin (cage activity experiment) or a fluorescent marker that had been injected immediately before activity (eccentric exercise experiment).

RESULTS

Obese mice exhibited evidence of increased myofiber membrane damage compared with lean mice after both normal cage activity and eccentric exercise indicating that myofiber membranes of obese mice are more susceptible to mechanical damage in general and that eccentric exercise exacerbates this effect.

CONCLUSIONS

These observations are consistent with the notion that obesity influences the biochemical and biomechanical properties of myofiber membranes.

Pseudo-ternary phase diagrams of aqueous mixtures of Quil A, cholesterol and phospholipid prepared by the lipid-film hydration method

Pseudo-ternary phase diagrams of the polar lipids Quil A, cholesterol (Chol) and phosphatidylcholine (PC) in aqueous mixtures prepared by the lipid film hydration method (where dried lipid film of phospholipids and cholesterol are hydrated by an aqueous solution of Quil A) were investigated in terms of the types of particulate structures formed therein. Negative staining transmission electron microscopy and polarized light microscopy were used to characterize the colloidal and coarse dispersed particles present in the systems. Pseudo-ternary phase diagrams were established for lipid mixtures hydrated in water and in Tris buffer (pH 7.4). The effect of equilibration time was also studied with respect to systems hydrated in water where the samples were stored for 2 months at 4 degrees C. Depending on the mass ratio of Quil A, Chol and PC in the systems, various colloidal particles including ISCOM matrices, liposomes, ring-like micelles and worm-like micelles were observed. Other colloidal particles were also observed as minor structures in the presence of these predominant colloids including helices, layered structures and lamellae (hexagonal pattern of ring-like micelles). In terms of the conditions which appeared to promote the formation of ISCOM matrices, the area of the phase diagrams associated with systems containing these structures increased in the order: hydrated in water/short equilibration period<hydrated in buffer/short equilibration period<hydrated in water/prolonged equilibration period. ISCOM matrices appeared to form over time from samples, which initially contained a high concentration of ring-like micelles suggesting that these colloidal structures may be precursors to ISCOM matrix formation. Helices were also frequently found in samples containing ISCOM matrices as a minor colloidal structure. Equilibration time and presence of buffer salts also promoted the formation of liposomes in systems not containing Quil A. These parameters however, did not appear to significantly affect the occurrence and predominance of other structures present in the pseudo-binary systems containing Quil A. Pseudo-ternary phase diagrams of PC, Chol and Quil A are important to identify combinations which will produce different colloidal structures, particularly ISCOM matrices, by the method of lipid film hydration. Colloidal structures comprising these three components are readily prepared by hydration of dried lipid films and may have application in vaccine delivery where the functionality of ISCOMs has clearly been demonstrated.

Less hydrophobic phosphatidylcholine species simplify biliary vesicle morphology, but induce bile metastability with a broad spectrum of crystal forms

Cholesterol crystallization in bile is affected by phosphatidylcholine (PtdCho) hydrophobicity. The aim of the present study was to determine whether PtdCho species modulate the metastable-labile limit and equilibrium solubility of cholesterol in the micellar phase of bile, thereby altering the distribution of cholesterol to biliary lipid carriers and thus influencing cholesterol crystallization. Supersaturated model bile (with a cholesterol saturation index of 2.0 and a total lipid concentration of 10 g/dl) was prepared with various PtdCho/(bile salt+PtdCho) ratios (0.1-0.5) using egg yolk or soya bean PtdCho. Subsequently, the following features were determined: metastable-labile limit, equilibrium solubility of cholesterol, metastable zone, and cholesterol crystallization process. Less hydrophobic PtdCho species destabilized bile cholesterol to induce rapid crystallization, because of a broad integrated metastable zone, whereas more hydrophobic species stabilized bile cholesterol with a less integrated metastable zone and thus retarded cholesterol crystallization. Cholesterol crystallization was accelerated by a decrease in the PtdCho/(bile salt+PtdCho) ratio, whereas the final nucleated crystal mass was increased by an increase in this ratio. With decreasing hydrophobicity of the PtdCho species, the intermixed micellar/vesicular concentration of bile salts decreased in association with less formation of vesicles and increased formation of micelles, and a variety of crystal forms were detected. In conclusion, PtdCho species directly influenced the cholesterol crystallization process in model bile by remodelling the bile mesophase, and also had an indirect influence by altering the balance between bile salt micelles and vesicles.

Less hydrophobic phosphatidylcholine species simplify biliary vesicle morphology, but induce bile metastability with a broad spectrum of crystal forms

Cholesterol crystallization in bile is affected by phosphatidylcholine (PtdCho) hydrophobicity. The aim of the present study was to determine whether PtdCho species modulate the metastable-labile limit and equilibrium solubility of cholesterol in the micellar phase of bile, thereby altering the distribution of cholesterol to biliary lipid carriers and thus influencing cholesterol crystallization. Supersaturated model bile (with a cholesterol saturation index of 2.0 and a total lipid concentration of 10 g/dl) was prepared with various PtdCho/(bile salt+PtdCho) ratios (0.1-0.5) using egg yolk or soya bean PtdCho. Subsequently, the following features were determined: metastable-labile limit, equilibrium solubility of cholesterol, metastable zone, and cholesterol crystallization process. Less hydrophobic PtdCho species destabilized bile cholesterol to induce rapid crystallization, because of a broad integrated metastable zone, whereas more hydrophobic species stabilized bile cholesterol with a less integrated metastable zone and thus retarded cholesterol crystallization. Cholesterol crystallization was accelerated by a decrease in the PtdCho/(bile salt+PtdCho) ratio, whereas the final nucleated crystal mass was increased by an increase in this ratio. With decreasing hydrophobicity of the PtdCho species, the intermixed micellar/vesicular concentration of bile salts decreased in association with less formation of vesicles and increased formation of micelles, and a variety of crystal forms were detected. In conclusion, PtdCho species directly influenced the cholesterol crystallization process in model bile by remodelling the bile mesophase, and also had an indirect influence by altering the balance between bile salt micelles and vesicles.

Is a role of phospholipase A(2) in cholesterol gallstone formation phospholipid species-dependent?

Phospholipase A(2) plays a role in cholesterol gallstone formation by hydrolyzing bile phospholipids into lysolecithin and free fatty acids. This study investigated its effects on cholesterol crystallization in model bile systems. Supersaturated model bile solutions with different cholesterol saturation indexes (1.2, 1.4, and 1.6) were prepared using cholesterol, taurocholate, and egg yolk phosphatidylcholine, soybean phosphatidylcholine, palmitoyl-oleoyl phosphatidylcholine, or palmitoyl-linoleoyl phosphatidylcholine. Then the effect of digestion of phosphatidylcholine by phospholipase A(2) on bile metastability was assessed by spectrophotometry and video-enhanced differential contrast microscopy. Addition of phospholipase A(2) caused the release of free fatty acids in a time-dependent manner. Cholesterol crystallization was enhanced by an increased crystal growth rate in model bile containing hydrophilic species such as soybean or palmitoyl-linoleoyl phosphatidylcholine, consisting predominantly of polyunsaturated fatty acids. Because phospholipase A(2) enhanced cholesterol crystallization in bile containing hydrophilic phosphatidylcholine species, but not hydrophobic phosphatidylcholine species, release of polyunsaturated fatty acids by hydrolysis may be responsible for such enhancement. Therefore, the role of phospholipase A(2) in cholesterol gallstone formation depends on the phospholipid species present in bile, so that phospholipid species selection during hepatic excretion is, in part, crucial to the cholesterol stone formation.

Partial replacement of bile salts causes marked changes of cholesterol crystallization in supersaturated model bile systems

Cholesterol crystallization is a key step in gallstone formation and is influenced by numerous factors. Human bile contains various bile salts having different hydrophobicity and micelle-forming capacities, but the importance of lipid composition to bile metastability remains unclear. This study investigated the effect of bile salts on cholesterol crystallization in model bile (MB) systems. Supersaturated MB systems were prepared with an identical composition on a molar basis (taurocholate/phosphatidylcholine/cholesterol, 152 mM:38 mM: 24 mM), except for partial replacement of taurocholate (10, 20, and 30%) with various taurine-conjugated bile salts. Cholesterol crystallization was quantitatively estimated by spectrophotometrically measuring crystal-related turbidity and morphologically scanned by video-enhanced microscopy. After partial replacement of taurocholate with hydrophobic bile salts, cholesterol crystallization increased dose-dependently without changing the size of vesicles or crystal morphology and the rank order of crystallization was deoxycholate>chenodeoxycholate>cholate (control MB). All of the hydrophilic bile salts (ursodeoxycholate, ursocholate and beta-muricholate) inhibited cholesterol precipitation by forming a stable liquid-crystal phase, and there were no significant differences among the hydrophilic bile-salt species. Cholesterol crystallization was markedly altered by partial replacement of bile salts with a different hydrophobicity. Thus minimal changes in bile-salt composition may dramatically alter bile lipid metastability.

Self-assembly of helical ribbons

The self-assembly of helical ribbons is examined in a variety of multicomponent enantiomerically pure systems that contain a bile salt or a nonionic detergent, a phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol. In almost all systems, two different pitch types of helical ribbons are observed: high pitch, with a pitch angle of 54 +/- 2 degrees, and low pitch, with a pitch angle of 11 +/- 2 degrees. Although the majority of these helices are right-handed, a small proportion of left-handed helices is observed. Additionally, a third type of helical ribbon, with a pitch angle in the range 30-47 degrees, is occasionally found. These experimental findings suggest that the helical ribbons are crystalline rather than liquid crystal in nature and also suggest that molecular chirality may not be the determining factor in helix formation. The large yields of helices produced will permit a systematic investigation of their individual kinetic evolution and their elastic moduli.

Role of bile salt hydrophobicity in distribution of phospholipid species to carriers in supersaturated model bile solutions

BACKGROUND

Phospholipid species modulate cholesterol-holding capacity and, therefore, regulate bile metastability.

METHODS

In this study, we investigated the effect of bile salt hydrophobicity on the distribution of phospholipids among lipid particles in supersaturated model bile solutions (total lipid concentration, 9 g/dL; taurocholate/phospholipid ratio 3.0, cholesterol saturation index 1.3), by using gel permeation chromatography.

RESULTS

With an increase of bile salt hydrophobicity in the elution buffer, the uptake of cholesterol and phospholipids into bile salt micelles was increased, associated with an increased cholesterol/phospholipid molar ratio of the vesicles. In contrast, there was an inverse correlation between the hydrophobicity of the phospholipid species in the vesicles and that of bile salts in the elution buffer, suggesting that hydrophobic bile salts induced preferential uptake of hydrophobic phospholipids into bile salt micelles, while less hydrophobic phospholipids, with a relatively low cholesterol-holding capacity, remained in the vesicles.

CONCLUSIONS

These data indicate that bile salt hydrophobicity regulates vesicular cholesterol metastability by modulating the hydrophobicity of phospholipids in vesicles, as well as the lipid distribution among various biliary lipid particles.

Quantitative assessment of comparative potencies of cholesterol-crystal-promoting factors: relation to mechanistic characterization

The crystallization of cholesterol is affected by various factors in bile. The present study evaluated the relative importance of cholesterol-nucleation-promoting factors and partially characterized the mechanisms of their action. Model biles with an identical relative composition of cholesterol, egg-yolk phosphatidylcholine and taurocholate, except for replacing phosphatidylcholine (5-20%) with dilinoleoyl-phosphatidylcholine or taurocholate (10-30%) with taurodeoxycholate. Cholesterol crystallization was quantitatively assessed spectrophotometrically and morphologically estimated by the laser-scattering diffraction analyser and video-enhanced microscopy in the absence and presence of concanavalin A-binding glycoprotein isolated from human bile. In a series of experiments, lipid distribution among particulate species was determined after isolation by FPLC. In all experiments, cholesterol crystallization was dose-dependently enhanced with a rank order of: concanavalin A-binding glycoprotein > dilinoleoyl - phosphatidyl choline> taurodeoxycholate. No morphological alteration was evident for vesicles and crystals, but the cholesterol/phospholipid ratio in vesicles was increased significantly by replacement with dilinoleoyl-phosphatidylcholine and excess cholesterol. A high proportion of relatively hydrophilic phosphatidylcholine species such as dilinoleoyl-phosphatidylcholine and excess cholesterol in bile cause a redistribution of cholesterol to increase a vesicular cholesterol/phospholipid ratio, eventually promoting cholesterol crystallization, whereas concanavalin A-binding glycoprotein acts via differing mechanisms.

The effects of phospholipid molecular species on cholesterol crystallization in model biles: the influence of phospholipid head groups

BACKGROUND/AIMS

Variations in the molecular species of biliary phospholipids have been shown to exert major effects on cholesterol solubility and carriers in model and human biles. The aim of this study was to explore systematically the effects of various phospholipid head groups on the cholesterol crystallization process in model biles.

METHODS

Three different control model biles were prepared using varying proportions of egg lecithin, cholesterol and Na taurocholate. In the test biles, 20% of the egg lecithin was replaced with synthetic phosphatidylserine, phosphatidylethanolamine, phosphatidylglycerol or phosphatidylcholine, keeping the phospholipid acyl chains and other biliary lipids constant in each experiment.

RESULTS

Phosphatidylserine and phosphatidylglycerol significantly prolonged the crystal observation time, from 2 days to 10 and 6 days, respectively (p<0.02), while phosphatidylethanolamine had little and phosphatidylcholine no effect. The crystal growth rate was significantly slowed down with 20% phospholipid replacement in the following order: phosphatidylglycerol >phosphatidylserine >phosphatidylethanolamine. The total crystal mass after 14 days, as measured by chemical analysis, was reduced by 59% with phosphatidylserine (p<0.05), and by 73% with phosphatidylglycerol (p<0.05); while phosphatidylethanolamine had little effect. The precipitable cholesterol crystal fractions after 14 days were significantly reduced with phosphatidylserine (54%) and phosphatidylglycerol (37%), but not with phosphatidylethanolamine or phosphatidylcholine.

CONCLUSIONS

Variations in the head groups of biliary phospholipids may markedly slow down the cholesterol crystallization process in model biles.

Increased saturation of the fatty acids in the sn-2 position of phospholipids reduces cholesterol crystallization in model biles

Changes in the molecular structure of biliary phospholipids were shown to have major effects on cholesterol solubility, carriers and crystallization in human and model biles. This study investigated systematically the effects of varying saturation of the phosphatidylcholine (PC) sn-2 fatty acid on the cholesterol crystallization process in 3 different model biles. Twenty % of the egg PC (EPC) in these biles were replaced by synthetic PC's with 16:0-18:0, 16:0-18:1, or 16:0-18:2 fatty acyl chains. With 18:0 in the sn-2 position, the crystal observation time (COT) was prolonged from 2 days in the control EPC solution to 14 days (p<0.05). The crystal growth rate (CGR) was reduced from 0.1 OD/day to unmeasurable levels, and the total crystal mass on day 14 decreased by 86%. The introduction of one (18:1), and two (18:2) double bonds in the sn-2 fatty acid rapidly reversed these effects. Ultracentrifugal analysis showed precipitable cholesterol as monohydrate crystals. In the 16:0-18:0 test solution, most of the precipitable cholesterol remained in the supersaturated multilamellar vesicles. Saturation of the biliary PC sn-2 fatty acyl chain prolongs the COT, slows the CGR, reduces the crystal mass, and extends cholesterol solubility in multilamellar vesicles. Desaturation of the sn-2 fatty acid reverses these effects.

Transcytotic vesicle fusion with canalicular membranes is modulated by phospholipid species: implications for biliary lipid secretion

Phospholipid species modulate bile metastability and the subselection of such species for biliary secretion occurs at the canalicular membrane. In this study, the role of phospholipid head groups and hydrophobic indices in transcytotic vesicle fusion with the canalicular membrane inner leaflet was investigated using rat canalicular membrane vesicles (CMV) and liposomes. The CMV were purified from Sprague-Dawley rat liver, and small unilamellar vesicles (SUV) of phosphatidylserine (PS), phosphatidylcholine (PC) and mixtures of PS/PC (1:1, 2:1 and 4:1) were labelled with 8 mol% of octadecyl rhodamine B chloride (R18). The PC species used in this study were egg yolk PC (EYPC), soybean PC (SBPC), dipalmitoyl PC (DPPC) and dilinoleoyl PC (DLPC). Fusion of SUV with CMV was initiated by the addition of a millimolar concentration of Ca2+ and the degree of fusion was estimated by the increase of R18 fluorescence. Ca(2+)-dependent fusion of SUV consisting of PS, and PS/PC (4:1) with CMV was observed (PS > PS/PC; 4:1), whereas no detectable fusion was evident between CMV and SUV of PC alone or PS/PC (1:1 or 2:1). The rank order of fusibility between CMV and SUV of PS/PC (4:1) containing various PC species was PS/DLPC > PS/SBPC > PS/EYPC > PS/DPPC. The hydrophobic index of PC as determined by high performance liquid chromatography (HPLC) was related closely to liposome fusibility (r = -0.88). These results suggest that transcytotic vesicle fusion with the canalicular membrane inner leaflet is regulated by the phospholipid hydrophobicity of the vesicles.

Influence of bile salts on molecular interactions between sphingomyelin and cholesterol: relevance to bile formation and stability

Bile salts enhance secretion of cholesterol into bile and its subsequent solubilization with phosphatidylcholine in mixed micelles. Sphingomyelin, a major structural lipid of the hepatocyte canalicular membrane, and disaturated phosphatidylcholines are known to impede nucleation of solid cholesterol crystals in supersaturated model systems. To understand these effects physico-chemically, we compared the influence of bile salts on interactions of cholesterol with natural sphingomyelins, as well as with dipalmitoyl and egg yolk phosphatidylcholines using various in vitro systems. Submicellar bile salts enhanced significantly bidirectional transfer of dehydroergosterol (a fluorescent cholesterol analog) between sphingomyelin and egg yolk phosphatidylcholine vesicles in the rank order taurocholate < tauroursodeoxycholate < taurodeoxycholate. Quasielastic light scattering of serially diluted sphingomyelin-taurocholate mixtures (1:1 molar ratio, 3 g/dl) revealed metastable temperature-dependent transitions between globular micelles, rod-shaped micelles and vesicles, suggesting that phase transitions under these experimental conditions were metastable only at temperatures below 37 degrees C. Ternary phase diagrams of all sphingomyelins and dipalmitoyl phosphatidylcholine with cholesterol and taurocholate (37 degrees C, 3 g/dl, 0.15 M NaCl) were identical. Compared to systems containing egg yolk phosphatidylcholine, the 1-phase micellar zone and 2- and 3-phase solid cholesterol crystal-containing zones were reduced markedly while the 2-phase zone with stable cholesterol-sphingomyelin liquid crystals was greatly expanded. Our results suggest that the high affinity of cholesterol for sphingomyelin is lost in the presence of bile salts. Our findings may be relevant to secretion of cholesterol into bile and to its inability to crystallize in the hepatocyte canalicular lumen or its surrounding membranes.

Inducing cholesterol precipitation from pig bile with beta-cyclodextrin and cholesterol dietary supplementation

BACKGROUND/METHODS

In this study, pigs fed for 3 weeks a well-balanced semi-purified diet enriched with 0.3% cholesterol and 0, 5 or 10% beta-cyclodextrin were proposed as new animal donors of gallbladder bile exhibiting different rates of cholesterol crystallization, in order to gain insight into the early mechanisms underlying cholesterol precipitation in vivo. The appearance and growth of cholesterol crystals were monitored in the incubated freshly collected gallbladder biles through light microscopy and concomitant time-sequential determination of crystallized cholesterol concentration, and interpreted in terms of the composition of the bile.

RESULTS

Although the concentration of total lipids and proteins and the relative proportions of bile acids, phospholipids, and cholesterol remained unchanged under beta-cyclodextrin, the cholesterol crystallization increased in the following order: 0<<10<5% beta-cyclodextrin. Concomitantly, the proportion of chenodeoxycholic acid in bile, and the hydrophobicity index of the biliary bile acid mixture increased in the following order: 0<5<10% beta-cyclodextrin (the same as reported elsewhere for the decrease in the antinucleating ApoA1), while sn-2 arachidonoyl biliary lecithins were specifically increased with 5% beta-cyclodextrin in the diet.

CONCLUSIONS

We hypothesized that lecithin molecular species may be the determinant factor in modulating high cholesterol crystallization rates in biles otherwise enriched with hydrophobic bile acids.

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.

Lecithin hydrophobicity modulates the process of cholesterol crystal nucleation and growth in supersaturated model bile systems

The present study was performed to determine whether the degree of lecithin hydrophobicity regulates bile metastability and, therefore, affects the process of cholesterol crystallization. Supersaturated model bile (MB) solutions were prepared with an identical composition on a molar basis (taurocholate/lecithin/cholesterol, 73:19.5:7.5; total lipid concentration 9 g/dl) except for the lecithin species; egg yolk phosphatidylcholine, soybean phosphatidylcholine, 1-palmitoyl-2-linoleoyl-sn-phosphatidylcholine, dilinoleoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine. Each MB solution was incubated and sequentially examined. Video-enhanced contrast microscopy demonstrated that the rate of vesicular aggregation and fusion correlated with the degree of lecithin hydrophobicity, and that the rate of cholesterol crystal nucleation correlated with the degree of lecithin hydrophilicity. In MBs containing less hydrophobic lecithin, needle-like crystals developed and transformed into mature plate-like crystals, whereas classical plate-like crystals were consistently observed in MBs composed of hydrophobic lecithin. Laser-diffraction particle size analysis demonstrated that the increase in lecithin hydrophobicity enlarged the vesicle dimension, enhancing its cholesterol-holding capacity. Correlation between vesicular cholesterol packing density and lecithin hydrophobicity suggests that the process of bile cholesterol nucleation and growth is regulated, in part, by acyl chain unsaturation in lecithin. Since the composition of biliary lecithins is responsive to dietary manipulations, this study provides new insights into the prevention of cholesterol gallstones.

Prevention of cholesterol cholelithiasis by dietary unsaturated fats in hormone-treated female hamsters

We examined the effect of diet on gallstone incidence and the composition of biliary phosphatidylcholines in methyltestosterone-treated female hamsters. These hamsters were fed a nutritionally adequate purified lithogenic diet containing 2% corn oil, 4% butterfat, 0.3% cholesterol, and 0.05% methyltestosterone, resulting in a cholesterol gallstone incidence of 86%. This incidence was lowered when mono- and polyunsaturated fats or fatty acids were added to the diet: 2.5% oleic acid resulted in total prevention of cholesterol cholelithiasis, 2.5% linoleic acid, and 4% safflower oil (78% linoleic acid content) reduced gallstone incidence to 26 and 8%, respectively. An additional 4% butterfat (29% oleic acid content) produced gallstones in 50% of the animals. At the end of the 6-wk feeding period, the bile of all hamsters was supersaturated with cholesterol. The major biliary phosphatidylcholine species in all groups were (sn-1-sn-2): 16:0-18:2, 16:0-18:1, 18:0-18:2, 16:0-20:4, and 18:2-18:2. The safflower oil- and linoleic acid-fed hamsters exhibited an enrichment of 16:0-18:2 (16-18%); added butterfat or oleic acid increased the proportion of 16:0-18:1 (9 and 25%, respectively). We conclude that the phosphatidylcholine molecular species in female hamster bile can be altered by dietary fats/fatty acids and that mono- and polyunsaturated fatty acids play a role in suppressing the induced cholelithiasis.

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.

Partial characterization of regulation of biliary lecithin hydrophobicity: association with organic anion-induced solute cholestasis in rats

We examined the effects of the depletion of bile salts and of the intravenous infusion of sodium taurocholate (STC) with or without bromosulphophthalein (BSP) in rats on the biliary secretion of lipids to clarify the regulatory mechanism(s). Each rat was equipped with a bile-duct cannula to collect bile. After the endogenous bile salt pool was depleted, STC was infused at a constant rate (160 nmol/min per 100 g body wt.) with or without BSP (50, 100, or 150 nmol/min per 100 g body wt.). BSP reduced the biliary secretion of cholesterol and phospholipids dose-dependently without affecting the secretion of bile salts (uncoupling phenomenon). Compared with the physiological and STC-infused condition, the biliary cholesterol/phospholipid ratio and saturated/unsaturated fatty acid ratio increased under the bile salts depletion and uncoupling phenomenon. Data indicate that the hydrophobicity of biliary lecithin increases with a decrease in the bile salt micelle capacity to induce biliary lipid secretion, resulting in a higher packing density of biliary vesicle. The cholesterol-holding capacity of the biliary vesicle is therefore enhanced during the depletion of bile salts and the uncoupling phenomenon.

Effects of long-term treatment with low-dose pravastatin on biliary lipid and bile acid composition in patients with nonfamilial hyperlipoproteinemia

We tested the possibility that pravastatin, a competitive inhibitor of hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase, would alter cholesterol saturation of gallbladder bile by decreasing its cholesterol saturation index and/or degree of fatty acyl chain unsaturation in lecithin. Eighteen patients with type IIa hyperlipoproteinemia were treated with pravastatin 10 mg/d for 12 months. Gallbladder bile samples were aspirated with a duodenal tube by stimulating gallbladder contraction with intramuscular administration of cerulein before and after treatment. Serum cholesterol level was significantly reduced by 20% after 3 months, and this level was maintained after 12 months. In contrast, the cholesterol saturation index of gallbladder bile was not altered after 3 months (1.52 +/- 0.20 v 1.70 +/- 0.24), but it decreased significantly after 12 months (0.95 +/- 0.11, P < .01). The degree of fatty acyl chain unsaturation tended to decrease, although this was not statistically significant except for the decrease in molar percent of linoleate after 3 months. These findings suggest that long-term treatment with an inhibitor of HMG CoA reductase improves bile lithogenicity even at a comparatively low dose, and can decrease the incidence and complications of cholesterol gallstones.