Roles of cholesterol and bile salts in the pathogenesis of gallbladder hypomotility and inflammation: cholecystitis is not caused by cystic duct obstruction

A large number of human and animal studies have challenged the hypothesis that cystic duct obstruction by gallstones causes cholecystitis. These studies suggest that lithogenic bile that can deliver high cholesterol concentrations to the gallbladder wall causes hypomotility and creates a permissive environment that allows normal concentrations of hydrophobic bile salts to inflame the mucosa and impair muscle function inhibiting gallbladder emptying. High concentrations of cholesterol increase its diffusion rates through the gallbladder wall where they are incorporated into the sarcolemmae of muscle cells by caveolin proteins. High caveolar cholesterol levels inhibit tyrosine-induced phosphorylation of caveolin proteins required to transfer receptor-G protein complexes into recycling endosomes. The sequestration of these receptor-G protein complexes in the caveolae results in fewer receptors recycling to the sarcolemmae to be available for agonist binding. Lower internalization and recycling of CCK-1 and other receptors involved in muscle contraction explain gallbladder hypomotility. PGE2 receptors involved in cytoprotection are similarly affected. Cells with a defective cytoprotection failed to inactivate free radicals induced by normal concentrations of hydrophobic bile salts resulting in chronic inflammation that may lead to acute inflammation. Ursodeoxycholic acid salts (URSO) block these bile salts effects thereby preventing the generation of free radicals in muscle cells in vitro and development of cholecystitis in the ligated common bile duct in guinea pigs in vivo. Treatment with URSO improves muscle contraction and reduces the oxidative stress in patients with symptomatic cholesterol gallstones by lowering cholesterol concentrations and blocking the effects of hydrophobic bile salts on gallbladder tissues.

Cholesterol synthesis inhibition distal to squalene upregulates biliary phospholipid secretion and counteracts cholelithiasis in the genetically prone C57L/J mouse

BACKGROUND AND AIMS

Newly synthesised cholesterol contributes poorly to biliary lipid secretion but may assume greater importance when the rate limiting enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) is upregulated. As this occurs in the gall stone susceptible C57L/J inbred mouse, we employed two cholesterol biosynthesis inhibitors, Tu 2208 and Ro 48-8071, potent inhibitors of squalene epoxidase and oxidosqualene-lanosterol cyclase, respectively, to assess their potential in preventing cholesterol cholelithiasis in the C57L/J mouse strain. Mice were fed a lithogenic diet comprising a balanced nutrient intake with 15% dairy fat, 1% cholesterol, and 0.5% cholic acid added.

METHODS

We determined gall stone phenotype, HMGR activity, biliary lipid secretion rates, and counterregulatory events in male C57L/J mice and gall stone resistant AKR treated with Tu 2208 (30-60 mg/kg/day) or Ro 48-8071 (30-100 mg/kg/day), while ingesting chow or the lithogenic diet.

RESULTS

Both agents reduced the gall stone prevalence rate from 73% to 17% in C57L/J mice, inhibited HMGR activity, and decreased hepatic cholesterol concentrations without appreciably influencing biliary cholesterol secretion. In C57L as well as AKR mice, both agents increased biliary phospholipid (which is mostly phosphatidylcholine) secretion rates and at the highest doses effectively reduced the biliary cholesterol saturation index.

CONCLUSIONS

Cholesterol biosynthesis inhibitors acting distally to squalene do not reduce biliary cholesterol secretion rates despite reductions in cholesterol biosynthesis and hepatocellular levels. However, they effectively prevent gall stone formation through stimulation of pathways that lead to enhanced biliary phospholipid secretion.

Enterohepatic cycling of bilirubin as a cause of 'black' pigment gallstones in adult life

In contrast to bile salts, which undergo a highly efficient enterohepatic circulation with multiple regulatory and physiologic functions, glucuronic acid conjugates of bilirubin are biliary excretory molecules that in health do not have a continuing biologic life. Intestinal absorptive cells are devoid of recapture transporters for bilirubin conjugates, and their large size and polarity prevent absorption by passive diffusion. However, unconjugated bilirubin, the beta-glucuronidase hydrolysis product of bilirubin glucuronides can be absorbed passively from any part of the small and large intestines. This can occur only if unconjugated bilirubin is kept in solution and does not undergo rapid bacterial reduction to form urobilinoids. Here we collect, and in some cases reinterpret, experimental and clinical evidence to show that in addition to the well-known occurrence in newborns, enterohepatic cycling of unconjugated bilirubin can reappear in adult life. This happens as a result of several common conditions, particularly associated with bile salt leakage from the small intestine, the most notable ileal dysfunction resulting from any medical or surgical cause. We propose that when present in excess, colonic bile salts solubilize unconjugated bilirubin, delay urobilinoid formation, prevent calcium complexing of unconjugated bilirubin and promote passive absorption of unconjugated bilirubin from the large intestine. Following uptake, reconjugation, and resecretion into bile, this source of 'hyperbilirubinbilia' may be the important pathophysiological risk factor for 'black' pigment gallstone formation in predisposed adult humans.

Mapping cholesterol gallstone susceptibility (Lith) genes in inbred mice

The individual risk for developing cholesterol gallstones in response to specific environmental factors is determined by complex genetics involving multiple genes. In this review, we introduce inbred mice as a model to localise and identify the murine genes that harbour cholesterol gallstone susceptibility alleles (Lith genes). These genes are associated with increased risk of gallstone formation when mice are fed a lithogenic diet containing cholesterol and cholic acid. We summarise the steps involved in localising the chromosomal regions that harbour Lith genes, focusing particularly on the initial step known as quantitative trait locus mapping, which employs breeding crosses of gallstone-susceptible and gallstone-resistant inbred mouse strains. Subsequent steps to narrow the chromosomal regions of the quantitative trait loci and identify the underlying Lith genes are outlined, with particular reference to the examples of Lith1 and Lith2, the first discovered quantitative trait loci associated with murine cholesterol cholelithiasis. We have now reported five quantitative trait loci for murine cholelithogenesis, which are officially named Lith1 through Lith5. Once the genes underlying these quantitative trait loci and other chromosomal loci from ongoing mouse crosses are identified and confirmed, the 'road-map' for discovery of orthologous human LITH genes will be available and, thereafter, their putative roles in cholesterol gallstone formation can be tested in selected human populations.

Genetic factors at the enterocyte level account for variations in intestinal cholesterol absorption efficiency among inbred strains of mice

Interindividual and interstrain variations in cholesterol absorption efficiency occur in humans and animals. We investigated physiological biliary and small intestinal factors that might determine variations in cholesterol absorption efficiency among inbred mouse strains. We found that there were significant differences in cholesterol absorption efficiency measured by plasma, fecal, and lymphatic methods: <25% in AKR/J, C3H/J, and A/J strains; 25-30% in SJL/J, DBA/2J, BALB/cJ, SWR/J, and SM/J strains; and 31-40% in C57L/J, C57BL/6J, FVB/J, and 129/SvJ strains. In (AKRxC57L)F1 mice, the cholesterol absorption efficiency (31 +/- 6%) mimicked that of the C57L parent (37 +/- 5%) and was significantly higher than in AKR mice (24 +/- 4%). Although biliary bile salt compositions and small intestinal transit times were similar, C57L mice displayed significantly greater bile salt secretion rates and pool sizes than AKR mice. In examining lymphatic cholesterol transport in the setting of a chronic biliary fistula, C57L mice displayed significantly higher cholesterol absorption rates compared with AKR mice. Because biliary and intestinal transit factors were accounted for, we conclude that genetic variations at the enterocyte level determine differences in murine cholesterol absorption efficiency, with high cholesterol absorption likely to be a dominant trait. This study provides baseline information for identifying candidate genes that regulate intestinal cholesterol absorption at the cellular level.

Phenotypic characterization of Lith genes that determine susceptibility to cholesterol cholelithiasis in inbred mice: soluble pronucleating proteins in gallbladder and hepatic biles

BACKGROUND/AIMS

Gallstone susceptibility is high in C57L inbred mice (males > females) and low in AKR mice, related to variant lithogenic (Lith) genes. We examined the relationship between biliary crystallization-promoting proteins and gallstone susceptibility.

METHODS

Biliary protein and lipid concentrations were determined at 0, 7,14, 21, 28 and 56 days on a lithogenic diet.

RESULTS

Protein and soluble mucin concentrations in gallbladder biles increased markedly in males, but remained low in females of both strains and correlated with the cholesterol saturation index (CSI). In all groups, IgA and IgM concentrations decreased initially, but increased at later stages. There were no consistent changes in IgG concentrations, but aminopeptidase-N levels were higher in AKR than in C57L. During the lithogenic diet period, the CSI was > or = 2 in C57L males, approximately 1.5 in AKR males, and 1 in females of both strains. Taurodeoxycholate and taurochenodeoxycholate rose sharply in C57L, but remained low in AKR.

CONCLUSIONS

Hydrophobic bile salts, cholesterol supersaturation, and possibly, high mucin concentrations are associated with gallstone formation. In vitro crystallization-promoting immunoglobulins and aminopeptidase-N do not appear to be major factors in murine gallstone pathogenesis, in line with the observation that genes encoding these proteins do not co-localize with any known Lith locus.

Chromosomal organization of candidate genes involved in cholesterol gallstone formation: a murine gallstone map

Epidemiologic and family studies indicate that cholesterol gallstone formation is in part genetically determined. The major contribution to our current understanding of gallstone genes derives from animal studies, particularly cross-breeding experiments in inbred mouse strains that differ in genetic susceptibility to cholesterol gallstone formation (quantitative trait loci mapping). In this review we summarize how the combined use of genomic strategies and phenotypic studies in inbred mice has proven to be a powerful means of dissecting the complex pathophysiology of this common disease. We present a "gallstone map" for the mouse, consisting of all genetic loci that have been identified to confer gallstone susceptibility as well as putative candidate genes. Translation of the genetic loci and genes between mouse and human predicts chromosomal regions in the human genome that are likely to harbor gallstone genes. Both the number and the precise understanding of gallstone genes are expected to further increase with rapid progress of the genome projects, and multiple new targets for early diagnosis and prevention of gallstone disease should become possible.

Spread monomolecular films of monohydroxy bile acids and their salts: influence of hydroxyl position, bulk pH, and association with phosphatidylcholine

Undissociated dihydroxy bile acids, alone or with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), lie with their long axes parallel to aqueous-lipid interfaces [Fahey, D. A., Carey, M. C., and Donovan, J. M. (1995) Biochemistry 34, 10886-10897]. To test the generality of this orientation, we used an automated Langmuir-Pockels surface balance to examine pressure-molecular area isotherms and dipole moments of insoluble monohydroxy bile acids and their salts, which are sparingly soluble because of their presumed high Krafft points. We studied lithocholic acid (LCA) (the natural 3alpha-OH isomer), glycolithocholic acid (GLCA) (its glycine conjugate), and the semisynthetic isomers, 7alpha-OH- and 12alpha-OH-cholanoic acids with and without POPC, at pH values ranging from 2 to 12. Monolayer collapse pressures increased sigmoidally with ionization, giving apparent pK values of 7.0-8.5 and implying a stronger affinity of the bile salt anions for the interface. At monolayer collapse, the molecular area of LCA was approximately 85 A(2) independent of pH, consistent with the steroid nucleus lying flat. In contrast, the interfacial area of 7-OH-cholanoic acid decreased from approximately 80 A(2) at pH 2 to approximately 40 A(2) above pH 9, consistent with a more vertical orientation and approximating 12-OH-cholanoic acid, which exhibited a molecular area of approximately 45 A(2) at all pH values. All monohydroxy bile acids condensed POPC monolayers more effectively at low than at high (ionized) pH. We conclude that the 3-OH group is crucial for anchoring bile acids and their salts to the aqueous interface, with all monohydroxy species condensing phospholipid membranes regardless of ionization state.

Quantitative trait loci mapping for cholesterol gallstones in AKR/J and C57L/J strains of mice

Quantitative trait locus (QTL) mapping was used to locate genes that determine the difference in cholesterol gallstone disease between the gallstone-susceptible strain C57L/J and the gallstone-resistant strain AKR/J. Gallstone weight was determined in 231 male (AKR x C57L) F(1) x AKR backcross mice fed a lithogenic diet containing 1% cholesterol, 0.5% cholic acid, and 15% butterfat for 8 wk. Mice having no stones and mice having the largest stones were genotyped at approximately 20-cM intervals to find the loci determining cholesterol gallstone formation. The major locus, Lith1, mapped near D2Mit56 and was confirmed by constructing a congenic strain, AK. L-Lith1(s). Another locus, Lith2, mapped near D19Mit58 and was also confirmed by constructing a congenic strain AK.L-Lith2(s). Other suggestive, but not statistically significant, loci mapped to chromosomes 6, 7, 8, 10, and X. The identification of these Lith genes will elucidate the pathophysiology of cholesterol gallstone formation.

Fluorocholesterols, in contrast to hydroxycholesterols, exhibit interfacial properties similar to cholesterol

We used an automated Langmuir-Pockels surface balance to characterize the air-water interfacial properties of cholesterol (CH) and its derivatives with hydrophilic OH and F substitutions at isologous sites on the sterol body or side chain. We studied 6-fluorocholesterol, 25-fluorocholesterol, 25,26,26,26,27,27,27-heptafluorocholesterol, 7alpha-hydroxycholesterol, 7beta-hydroxycholesterol, 25-hydroxycholesterol and 27-hydroxycholesterol, alone and in mixtures with 1-palmitoyl-2-oleoyl-sn-3-glycero-phosphocholine (POPC). Pressure;-area isotherms of the fluorocholesterols were essentially indistinguishable from CH and all condensed POPC monomolecular layers (monolayers) to variable degrees. Both nucleus-substituted hydroxycholesterols formed expanded monolayers, with lift-offs from baseline 22-26 A(2)/molecule larger than CH, suggesting interfacial tilting; furthermore, in binary mixtures, they condensed POPC monolayers less than CH. In contrast, the side chain hydroxylated CHs were oriented horizontally in the interface at large molecular areas, and became vertical below 140 A(2)/molecule with the side chain-OH rather than 3-OH group anchored in the subphase, as evidenced by low collapse pressures and smaller molecular areas than CH. Both side chain hydroxycholesterols expanded POPC monolayers at molar ratios <30%, but induced condensation with higher ratios, suggesting that OH-acyl chain (POPC) repulsion is superceded at higher mole fractions by lateral phase separation and intersteroidal H-bonding. These studies predict that fluorocholesterols should exhibit intramembrane spatial occupancy nearly identical to CH, whereas nucleus and especially side chain hydroxycholesterols will perturb membrane lipid packing notably.

High-resolution maps of the murine Chromosome 2 region containing the cholesterol gallstone locus, Lith1

The Lith1 region on Chromosome (Chr) 2 contains a gene that markedly affects the prevalence of cholesterol gallstones in inbred mice. We report the high-resolution genetic and radiation hybrid maps of the chromosomal region surrounding Lith1, using three resources: a DNA panel from 188 progeny from two reciprocal backcrosses between C57BL/6 and Mus spretus inbred strains; 423 progeny of an N4 generation from backcrossing the susceptible C57L/J alleles at Lith1 into the resistant AKR/J strain; and the newly developed hamster-mouse T31 radiation hybrid panel. We mapped 17 microsatellite markers in the D2Mit182 to D2Mit14 region and two candidate genes for Lith1, the canalicular bile salt export pump (Bsep) also known as sister of P-glycoprotein (Spgp) and the low-density-lipoprotein-receptor-related gene megalin (Gp330). Both genetic maps were in agreement and ordered the microsatellite markers into a 10.4 +/- 1.5 cM region. The high-resolution physical map revealed ordering of microsatellite markers and relative distances between markers in almost complete agreement with the genetic maps. Mapping of Bsep revealed its location on Chr 2, homologous to the human chromosomal position (Nature Genet 20, 233-238, 1998). The radiation hybrid results also provided the highest resolution of the area containing the two candidate genes, which both mapped in the Lith1 region with close linkage, being separated by a distance of only 15 cR(3000). The total radiation hybrid map length of the region between D2Mit182 and D2Mit14 was 326 cR(3000), suggesting that 31 cR(3000) is equivalent to 1 cM in this region of Chr 2.

Phenotypic characterization of lith genes that determine susceptibility to cholesterol cholelithiasis in inbred mice. Pathophysiology Of biliary lipid secretion

The inbred C57L strain but not the AKR strain of mice carry Lith genes that determine cholesterol gallstone susceptibility. When C57L mice are fed a lithogenic diet containing 15% fat, 1% cholesterol, and 0.5% cholic acid, gallbladder bile displays rapid cholesterol supersaturation, mucin gel accumulation, increases in hydrophobic bile salts, and rapid phase separation of solid and liquid crystals, all of which contribute to the high cholesterol gallstone prevalence rates (D. Q-H. Wang, B. Paigen, and M. C. Carey. J. Lipid Res. 1997. 38: 1395;-1411). We have now determined the hepatic secretion rates of biliary lipids in fasting male and female C57L and AKR mice and the intercross (C57L x AKR)F(1) before and at frequent intervals during feeding the lithogenic diet for 56 days. Bile flow and biliary lipid secretion rates were measured in the first hour of an acute bile fistula and circulating bile salt pool sizes were determined by the "washout" technique after cholecystectomy. Compared with AKR mice, we found that i) C57L and F(1) mice on chow displayed significantly higher secretion rates of all biliary lipids, and larger bile salt pool sizes, as well as higher bile salt-dependent and bile salt-independent flow rates; ii) the lithogenic diet further increased biliary cholesterol and lecithin outputs, but bile salt outputs remained constant. Biliary coupling of cholesterol to lecithin increased approximately 30%, setting the biophysical conditions necessary for cholesterol phase separation in the gallbladder; and iii) no gender differences in lipid secretion rates were noted but male mice exhibited significantly more hydrophobic bile salt pools than females. We conclude that in gallstone-susceptible mice, Lith genes determine increased outputs of all biliary lipids but promote cholesterol hypersecretion disproportionately to lecithin and bile salt outputs thereby inducing lithogenic bile formation.

Phenotypic characterization of Lith genes that determine susceptibility to cholesterol cholelithiasis in inbred mice: integrated activities of hepatic lipid regulatory enzymes

There is no consensus whether hepatic lipid regulatory enzymes play primary or secondary roles in cholesterol cholelithiasis. We have used inbred mice with Lith genes that determine cholesterol gallstone susceptibility to evaluate the question. We studied activities of regulatory enzymes in cholesterol biosynthesis (HMG-CoA reductase), cholesterol esterification (acyl-CoA:cholesterol acyltransferase) and the "neutral" (cholesterol 7alpha-hydroxylase) and "acidic" (sterol 27-hydroxylase) pathways of bile salt synthesis in strains C57L/J and SWR/J as well as recombinant inbred (AKXL-29) mice, all of which have susceptible Lith alleles, and compared them to AKR/J mice with resistant Lith alleles. We determined hepatic enzyme activities of male mice before and at frequent intervals during feeding a lithogenic diet (15% dairy fat, 1% cholesterol, 0.5% cholic acid) for 12 weeks. Basal activities on chow show significant genetic variations for HMG-CoA reductase, sterol 27-hydroxylase, and acyl-CoA: cholesterol acyltranferase, but not for cholesterol 7alpha-hydroxylase. In response to the lithogenic diet, activities of the regulatory enzymes in the two bile salt synthetic pathways are coordinately down-regulated and correlate inversely with prevalence rates of cholesterol crystals and gallstones. Compared with gallstone-resistant mice, significantly higher HMG-CoA reductase activities together with lower activities of both bile salt synthetic enzymes are hallmarks of the enzymatic phenotype in mice with susceptible Lith alleles. The most parsimonious explanation for the multiple enzymatic alterations is that the primary Lith phenotype induces secondary events to increase availability of cholesterol to supply the sterol to the hepatocyte canalicular membrane for hypersecretion into bile.

Enterohepatic cycling of bilirubin: a putative mechanism for pigment gallstone formation in ileal Crohn's disease

BACKGROUND & AIMS

Patients with ileal disease, bypass, or resection are at increased risk for developing gallstones. In ileectomized rats, bilirubin secretion rates into bile are elevated, most likely caused by increased colonic bile salt levels, which solubilize unconjugated bilirubin, prevent calcium complexing, and promote its absorption and enterohepatic cycling. The hypothesis that ileal disease or resection engenders the same pathophysiology in humans was tested.

METHODS

Sterile gallbladder bile samples were obtained intraoperatively from 29 patients with Crohn's disease and 19 patients with ulcerative colitis. Bilirubin, total calcium, biliary lipids, beta-glucuronidase activities, and cholesterol saturation indices in bile were measured, and markers of hemolysis and ineffective erythropoiesis in blood were assessed.

RESULTS

Bilirubin conjugates, unconjugated bilirubin, and total calcium levels were increased 3-10-fold in bile of patients with ileal disease and/or resection compared with patients with Crohn's colitis or ulcerative colitis. Biliary bilirubin concentrations correlated positively with the anatomic length and duration of ileal disease. Endogenous biliary beta-glucuronidase activities were comparable in all groups, and both the hemogram and serum vitamin B12 levels were normal.

CONCLUSIONS

This study establishes that increased bilirubin levels in bile of patients with Crohn's disease are caused by lack of functional ileum, supporting the hypothesis that enterohepatic cycling of bilirubin occurs.

Cryoelectron microscopy of a nucleating model bile in vitreous ice: formation of primordial vesicles

Because gallstones form so frequently in human bile, pathophysiologically relevant supersaturated model biles are commonly employed to study cholesterol crystal formation. We used cryo-transmission electron microscopy, complemented by polarizing light microscopy, to investigate early stages of cholesterol nucleation in model bile. In the system studied, the proposed microscopic sequence involves the evolution of small unilamellar to multilamellar vesicles to lamellar liquid crystals and finally to cholesterol crystals. Small aliquots of a concentrated (total lipid concentration = 29.2 g/dl) model bile containing 8.5% cholesterol, 22.9% egg yolk lecithin, and 68.6% taurocholate (all mole %) were vitrified at 2 min to 20 days after fourfold dilution to induce supersaturation. Mixed micelles together with a category of vesicles denoted primordial, small unilamellar vesicles of two distinct morphologies (sphere/ellipsoid and cylinder/arachoid), large unilamellar vesicles, multilamellar vesicles, and cholesterol monohydrate crystals were imaged. No evidence of aggregation/fusion of small unilamellar vesicles to form multilamellar vesicles was detected. Low numbers of multilamellar vesicles were present, some of which were sufficiently large to be identified as liquid crystals by polarizing light microscopy. Dimensions, surface areas, and volumes of spherical/ellipsoidal and cylindrical/arachoidal vesicles were quantified. Early stages in the separation of vesicles from micelles, referred to as primordial vesicles, were imaged 23-31 min after dilution. Observed structures such as enlarged micelles in primordial vesicle interiors, segments of bilayer, and faceted edges at primordial vesicle peripheries are probably early stages of small unilamellar vesicle assembly. A decrease in the mean surface area of spherical/ellipsoidal vesicles was correlated with the increased production of cholesterol crystals at 10-20 days after supersaturation by dilution, supporting the role of small unilamellar vesicles as key players in cholesterol nucleation and as cholesterol donors to crystals. This is the first visualization of an intermediate structure that has been temporally linked to the development of small unilamellar vesicles in the separation of vesicles from micelles in a model bile and suggests a time-resolved system for further investigation.

No pathophysiologic relationship of soluble biliary proteins to cholesterol crystallization in human bile

This study explores the pathophysiologic effects of soluble biliary glycoproteins in comparison to mucin gel and cholesterol content on microscopic crystal and liquid crystal detection times as well as crystallization sequences in lithogenic human biles incubated at 37 degrees C. Gallbladder biles from 13 cholesterol gallstone patients were ultracentrifuged and microfiltered (samples I). Total biliary lipids were extracted from portions of samples I, and reconstituted with 0.15 m NaCl (pH 7.0) (samples II). Portions of samples II were supplemented with purified concanavalin A-binding biliary glycoproteins (final concentration = 1 mg/mL) (samples III), or mucin gel (samples IV), respectively, isolated from the same cholesterol gallstone biles. Samples V consisted of extracted biliary lipids from uncentrifuged and unfiltered bile samples reconstituted with 0.15 m NaCl (pH 7.0). Analytic lipid compositions of samples I through IV were identical for individual biles but, as anticipated, samples V displayed significantly higher cholesterol saturation indexes. Detection times of cholesterol crystals and liquid crystals were accelerated in the rank order of samples: IV > V > I = II = III, indicating that total soluble biliary glycoproteins in pathophysiologic concentration had no appreciable effect. Crystallization sequences (D. Q-H. Wang and M. C. Carey. J. Lipid Res. 1996. 37: 606-630; and 2539-2549) were similar among samples I through V. Crystal detection times and numbers of solid cholesterol crystals were accelerated in proportion to added mucin gel and the cholesterol saturation of bile only. For pathophysiologically relevant conditions, our results clarify that mucin gel and cholesterol content, but not soluble biliary glycoproteins, promote cholesterol crystallization in human gallbladder bile.

Cholic acid aids absorption, biliary secretion, and phase transitions of cholesterol in murine cholelithogenesis

Cholic acid is a critical component of the lithogenic diet in mice. To determine its pathogenetic roles, we fed chow or 1% cholesterol with or without 0.5% cholic acid to C57L/J male mice, which because of lith genes have 100% gallstone prevalence rates. After 1 yr on the diets, we measured bile flow, biliary lipid secretion rates, hepatic cholesterol and bile salt synthesis, and intestinal cholesterol absorption. After hepatic conjugation with taurine, cholate replaced most tauro-beta-muricholate in bile. Dietary cholic acid plus cholesterol increased bile flow and biliary lipid secretion rates and reduced cholesterol 7alpha-hydroxylase activity significantly mostly via deoxycholic acid, cholate's bacterial 7alpha-dehydroxylation product but did not downregulate cholesterol biosynthesis. Intestinal cholesterol absorption doubled, and biliary cholesterol crystallized as phase boundaries shifted. Feeding mice 1% cholesterol alone produced no lithogenic or homeostatic effects. We conclude that in mice cholic acid promotes biliary cholesterol hypersecretion and cholelithogenesis by enhancing intestinal absorption, hepatic bioavailability, and phase separation of cholesterol in bile.

Sphingomyelins of rat liver: biliary enrichment with molecular species containing 16:0 fatty acids as compared to canalicular-enriched plasma membranes

We harvested canalicular-enriched plasma membranes of hepatocytes and collected fistula bile from male rats and isolated the sphingomyelins. Following sphingomyelinase hydrolysis, we identified the sphingomyelin molecular species on the basis of their benzoylated ceramide derivatives employing high performance liquid chromatography. Sphingomyelin constitutes </=3% of total biliary phospholipids (which are mostly sn-1 16:0 long-chain phosphatidylcholines) and approximately 30% of canalicular-enriched membranes. In both cases, the principal molecular species were composed of 16:0, 18:0, 20:0, 22:0, 23:0, 24:0, 24:1 and 24:2 fatty acid classes. However, the 16:0 fatty acid species was enriched in biliary sphingomyelin to a significantly greater degree than in sphingomyelins of canalicular-enriched plasma membranes (46% vs. 25% of total). We argue a physical-chemical case for laterally separated domains of very long chain sphingomyelins on the exoplasmic leaflet of the canalicular membrane. We bolster our hypothesis by the likelihood that the least hydrophobic, e.g., 16:0 sphingomyelin molecular species, are miscible with biliary phosphatidylcholines, and are secreted into bile. Laterally separated domains of very long chain sphingomyelins on the exoplasmic leaflet of the canalicular membrane could provide a means of sequestering cholesterol molecules prior to secretion into bile.

Sterol carrier protein 2 participates in hypersecretion of biliary cholesterol during gallstone formation in genetically gallstone-susceptible mice

In inbred mice, susceptibility to cholesterol gallstone disease is conferred by Lith genes, which in part promote hypersecretion of cholesterol into bile in response to a high-fat/cholesterol/cholic acid (lithogenic) diet. Because cytosolic sterol carrier protein 2 (SCP2) is believed to participate in cellular cholesterol trafficking and is elevated in the liver cytosol of cholesterol gallstone patients, we defined the hepatic expression of SCP2 during cholesterol gallstone formation in gallstone-susceptible C57L and gallstone-resistant AKR mice fed the lithogenic diet. Steady-state cytosolic SCP2 levels in C57L, but not AKR mice increased as a function of time and were correlated positively with biliary cholesterol hypersecretion, cholesterol saturation indices of gall-bladder biles and the appearance of liquid and solid cholesterol crystals leading to gallstone formation. Steady-state mRNA levels increased co-ordinately, consistent with regulation of SCP2 expression at the transcriptional level. Our results suggest that overexpression of SCP2 contributes to biliary cholesterol hypersecretion and the pathogenesis of gallstones in genetically susceptible mice. Because of the different chromosomal localizations of the Lith and Scp2 genes, we postulate that Lith genes control SCP2 expression indirectly.

Laser light scattering evidence for a common wormlike growth structure of mixed micelles in bile salt- and straight-chain detergent-phosphatidylcholine aqueous systems: relevance to the micellar structure of bile

We employed quasielastic and static light scattering to measure apparent values of the mean hydrodynamic radii (Rh)app, molecular weights (Mapp), and radii of gyration (Rg)app in solutions containing mixed micelles composed of bile salts (cholate and taurochenodeoxycholate, both cholanoyl derivatives) and the glycoacyl chain detergent, octyl glucoside, with egg yolk phosphatidylcholine (EYPC) as functions of total lipid concentration (0.1-10 g/dL), EYPC/detergent molar ratio (0-1.2), and ionic strength (0.15-0.4 M NaCl) at 20 degreesC and 1 atm. As the mixed micellar phase boundaries were approached by dilution, (Rh)app, Mapp, and (Rg)app values increased markedly by up to 20-fold. For each micellar system, the scaling ratios (Rh)app/Mapp1/2 and (Rg)app/(Rh)app remained essentially constant at 0.018 nm/(g/mol)1/2 and 1.5 (dimensionless), respectively, despite large variations in total lipid concentration, detergent molecular species, and ionic strength. Refined data analysis is inconsistent with a flat "mixed-disc" model for bile salt-EYPC micelles [Mazer, N. A., Benedek, G. B., and Carey, M. C. (1980) Biochemistry 19, 601] and octyl glucoside-EYPC micelles principally because the numerical value of (Rh)app/Mapp1/2 corresponds to a hypothetical disk thickness of approximately 1 nm, which is 4-fold smaller than the bimolecular width of EYPC molecules, and for a disk, (Rg)app/(Rh)app ratios should be close to 1 at low total lipid concentrations. Assuming disc-shaped micelles, we show that intermicellar excluded volume interactions would have only a minor effect on Mapp and cannot account for the unrealistic disk thickness. Instead, locally cylindrical, semiflexible wormlike micelles of diameter d = 4 nm and persistence length xip = 17 nm in solution are compatible with the observed (Rh)app/Mapp1/2 and (Rg)app/(Rh)app values when intermicellar excluded-volume interactions are considered. With EYPC/taurochenodeoxycholate = 0.6 and EYPC/cholate = 1.0 in 0.15 M NaCl, independent micelles grow upon dilution and use of the second virial coefficient [Egelhaaf, S. U., and Schurtenberger, P. (1994) J. Phys. Chem. 98, 8560] is adequate for estimating micellar weights. The systems EYPC/cholate = 1.0 in 0.4 M NaCl, EYPC/cholate = 1.2 in 0.15 M NaCl, and EYPC/octyl glucoside = 0.13 in 0.15 M NaCl all form highly overlapping, semidilute polymer solutions, which mimic the observed scaling ratios. In such semidilute systems, use of the second virial coefficient alone to account for intermicellar interactions is inadequate for estimating micellar weights. The results of the present study, in combination with locations of known phase boundaries of the ternary bile salt-EYPC-water phase diagram at high dilution, suggest that elongation, as well as entanglement of wormlike mixed micelles may occur at concentrations approaching the micellar phase limit.

Ursodeoxycholic acid and cholesterol induce enterohepatic cycling of bilirubin in rodents

BACKGROUND & AIMS

Oral administration of ursodeoxycholic acid (UDCA) and cholesterol causes bile salt malabsorption; the former by competition for and the latter by down-regulation of ileal bile acid transporters. Because ileectomy in rats induces enterohepatic cycling of bilirubin, the hypothesis that dietary steroids might have the same effect was tested.

METHODS

Male inbred C57L/J mice and Sprague-Dawley rats were fed low doses of UDCA, chenodeoxycholic acid (CDCA), or cholesterol added to laboratory chow with simultaneous chow-fed controls. After 1 week (mice) or 2 weeks (rats), indices of bile salt malabsorption and enterohepatic cycling of bilirubin were measured, including bilirubin secretion rates into bile, serum and intestinal bilirubin and bile salt levels, and urobilinogen levels in cecum, large intestine, and feces.

RESULTS

Dietary UDCA and cholesterol, but not CDCA, significantly increased bilirubin secretion rates into bile. In UDCA-fed mice, gallbladder biles contained increased levels of bilirubin conjugates and unconjugated bilirubin, and in 60%, granules of amorphous calcium bilirubinate precipitated. Dietary cholesterol and bile acids, particularly UDCA, increased cecal bile salt levels, unconjugated bilirubin and urobilinogen concentrations, and decreased fecal bilirubin outputs, consistent with colonic absorption.

CONCLUSIONS

By causing bile salt malabsorption, dietary UDCA and cholesterol induce enterohepatic cycling of bilirubin.

Evidence for an ATP-independent long-chain phosphatidylcholine translocator in hepatocyte membranes

Transport of phosphatidylcholine (PC) molecules across canalicular plasma membranes of the liver is essential for their secretion into bile. To test for evidence of protein-mediated translocation of natural long-chain PCs, we investigated whether hepatocyte membrane subfractions reconstituted into proteoliposomes promoted transmembrane translocation of radiolabeled PCs. Translocation of PC molecules in proteoliposomes was measured by an assay that employed multilamellar acceptor vesicles and the specific PC transfer protein purified from liver. As inferred from the percentage of radiolabel removed from proteoliposomes, facilitated PC translocation occurred in microsomes and canalicular and basolateral plasma membranes from rat liver but not in erythrocyte ghosts, microsomes, homogenates of COS and H35 cells, or Xenopus laevis oocytes. Heat denaturation in the presence of 2-mercaptoethanol and Pronase digestion of solubilized membrane proteins inhibited translocation. In contrast to the mdr2 gene product (Mdr2), which promotes ATP-dependent, verapamil-inhibitable PC translocation, ATP did not enhance and verapamil failed to block PC translocation. These data support the possibility that an ATP-independent PC translocator, possibly distinct from Mdr2, may be present in hepatocyte canalicular plasma membranes.

Phenotypic characterization of Lith genes that determine susceptibility to cholesterol cholelithiasis in inbred mice: physical-chemistry of gallbladder bile

Lith genes control susceptibility to cholesterol gallstone formation in inbred strains of mice on a lithogenic diet containing high fat, high cholesterol and 0.5% cholic acid. Our study defines the physical-chemical phenotypes of C57L, AKR, and (C57L x AKR) F1 mouse gallbladder biles during 56 days on the lithogenic diet. We found enhanced cholesterol supersaturation, accumulation of mucin gel, and larger gallbladders in all C57L and F1 mice, as well as more frequent gallstone formation in male C57L and F1 mice (80%) compared to females (40%) or AKR mice (15%). In male C57L and F1 mice, mucin gel accumulated at 3 days, followed by cholesterol supersaturation and phase separation of liquid crystals, solid monohydrate crystals, and, in 43% of mice, anhydrous cholesterol crystals; whereas, in females, phase separations were delayed 2 to 9 days, and anhydrous crystals did not form. In AKR mice, cholesterol supersaturation and phase separations were infrequent and delayed, and gender did not influence the phenotype. Taurocholate invariably replaced endogenous bile salts, especially tauro-beta-muricholate, with crystallization sequences matching taurocholate-containing model bile systems. We conclude: i) Lith genes determine biliary cholesterol supersaturation, mucin gel accumulation, gallbladder size, phase-separation, and prevalence of cholesterol gallstones. ii) Identical phenotypes in C57L and F1 mice indicate susceptibility to cholesterol gallstones is genetically dominant, favoring males 2:1. iii) Mucin gel accumulation, crystallization, and stone formation are rare in AKR mice. This definition of the physical chemistry of lithogenesis should aid in further elucidation of the Lith genes and the proteins they encode.

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.

Characterization of crystallization pathways during cholesterol precipitation from human gallbladder biles: identical pathways to corresponding model biles with three predominating sequences

In model biles, five crystallization sequences are present as functions of bile salt/lecithin (egg yolk) ratio and their positions on phase diagrams are influenced by bile salt hydrophobicity, temperature, and total lipid concentration (D. Q-H. Wang and M.C. Carey. J. Lipid Res. 1996.37: 606-630). To determine whether the same pathways occur ex vivo during cholesterol precipitation from human gallbladder biles, we examined 22 cholesterol gallstone (CSI = 1.56 +/- 0.26), 4 pigment gallstone (0.69 +/- 0.06), and 4 control biles (0.85 +/- 0.22) by microscopy and lipid analytic techniques for 30 days. Temperature was varied (4-45 degrees C) to move relative compositions into adjacent pathways or supersaturated zones to test whether the same bile could be forced to crystallize in different sequences. Sequences in native bile were identical to those in model systems composed of mixed bile salts-lecithin-cholesterol mixtures, and three corresponding pathways (B, C, D; op. cit.) were observed at 37 degrees C. With increasing lecithin content, we found i) B: plate-like cholesterol monohydrate crystals appeared before arc-shaped (putatively anhydrous cholesterol) crystals which transformed via helices and tubules into plate-like crystals and no liquid crystals formed; ii) C: lamellar liquid crystals, typified by birefringent multilamellar vesicles, were detected before cholesterol monohydrate crystals, and subsequently arc, helical and tubular crystals appeared; and iii) D: precipitation of lamellar liquid crystals was followed by cholesterol monohydrate crystals and no arc crystals were detected. Added EDTA prevented calcium bilirubinate formation, but crystallization sequences in these biles were identical to those without EDTA. We conclude that i) cholesterol crystallization pathways and sequences in human gallbladder biles are identical to model biles matched for appropriate physical-chemical conditions; ii) three of the five sequences observed in model biles were found in native bile; and iii) calcium bilirubinates neither promote biliary cholesterol crystallization nor influence crystal growth.

The Louisiana State University Comprehensive Epilepsy Program: procedures and outcomes

The first comprehensive epilepsy surgery center in Louisiana was established in 1990 at the Louisiana State University Medical Center in New Orleans by the Departments of Neurology and Neurosurgery. The center performs a wide variety of diagnostic tests essential for the medical and surgical treatment of epilepsy including EEG and video monitoring, quantitative hippocampal MRI volumetry, ictal SPECT brain scanning, intracranial evoked potential and subdural stimulation functional mapping, neuropsychological evaluations, and intracarotid amobarbital (Wada) language and memory localization. Surgical interventions include (1) the placement of subdural strip and grid electrodes, depth electrodes, and foramen ovale electrodes, (2) temporal lobectomies, and (3) frontal, temporal, parietal, and occipital lobe resections. From August 1990 through October 1995 41 patients with medically intractable seizures underwent neurosurgical procedures for epilepsy. Thirty-five patients had resective surgery, while six had only intracranial monitoring by subdural or intracerebral electrodes. The surgical outcomes thus far compare favorably with those of other established centers in North America.

Bilirubin cycles enterohepatically after ileal resection in the rat

BACKGROUND & AIMS

Patients with ileal disease, resection, or bypass are at increased risk of developing pigment gallstones, but the pathophysiological mechanisms are unknown. The aim of this study was to test the hypothesis that ileectomy induces enterohepatic cycling of bilirubin.

METHODS

Ileectomy or sham operation was performed in adult male Sprague-Dawley rats with the following control procedures: no operation, ileal transection, proximal or distal jejunectomy, ileocolonic transposition, and ileocecectomy. Bilirubin and bile salt secretion rates were measured after bile duct cannulation performed 3-11 days after intestinal surgery. Also measured were bilirubin and bile salt concentrations in the colon as well as indices of hemolysis in blood.

RESULTS

Compared with controls, bilirubin secretion rates were increased significantly 3-5 days after ileectomy, distal jejunectomy, ileocolonic transposition, and ileocecectomy, with no hemolysis occurring. Bile salt secretion rates also increased significantly after ileectomy but decreased markedly with prevention of coprophagy, whereas bilirubin secretion rates remained elevated. By 8-11 days after surgery, intestinal adaptation normalized bile salt reabsorption, and hypersecretion of bilirubin was abolished. Colonic levels of unconjugated bilirubin and bile salts were increased fivefold and eightfold respectively in ileectomized animals, but unconjugated bilirubin levels remained normal in bile.

CONCLUSIONS

These results are consistent with the hypothesis that enterohepatic cycling of bilirubin occurs with bile salt malabsorption.

Complete mapping of crystallization pathways during cholesterol precipitation from model bile: influence of physical-chemical variables of pathophysiologic relevance and identification of a stable liquid crystalline state in cold, dilute and hydrophilic bile salt-containing systems

Using complementary physical-chemical techniques we defined five different crystallization pathways as functions of time (30 days) and increasing lecithin (egg yolk) content in pathophysiologically relevant model biles super-saturated (cholesterol saturation indices, 1.2 - 2.7) by dilution of approximately equal to 29 g/dl bile salt-lecithin-cholesterol micellar solutions. As evidenced by quasi-elastic light-scattering spectroscopy, supersaturation was heralded by the appearance of unilamellar vesicles. With the lowest lecithin contents, arc-like crystals with habit and density (d 1.030 g/mL) consistent with anhydrous cholesterol appeared first and evolved via helical and tubular crystals to form plate-like cholesterol monohydrate crystals (d 1.045 g/mL). With higher lecithin fractions, cholesterol monohydrate crystals appeared earlier than arc and other transitional crystals. With typical physiological lecithin contents, early liquid crystals (d 1.020 g/mL) were followed by cholesterol monohydrate crystals and subsequent appearances of arc and other intermediate crystals. With higher lecithin contents, liquid crystals were followed by cholesterol monohydrate crystals only, and at the highest lecithin mole fractions, liquid crystals appeared that did not generate solid crystals. Added calcium increased solid crystal number in proportion to its concentration (5 - 20 mM) but did not influence appearance times, crystallization pathways, or micellar cholesterol solubilities. Decreases in temperature (37 degrees --> 4 degrees C), total lipid concentration (7.3 --> 2.4 g/dL), and bile salt hydrophobicity (3 alpha, 12 alpha --> 3 alpha, 7 alpha, 12 alpha --> 3 alpha, 7 beta hydroxylated taurine conjugates) progressively shifted all crystallization pathways to lower lecithin contents, retarded crystallization, and decreased micellar cholesterol solubilities. The lecithin content of mother biles decreased markedly during crystallization especially where liquid crystals were a coexisting phase at equilibrium. This systematic study provides a framework for understanding cholesterol crystallization in human and animal biles and for examining factors that influence the kinetics of phase separation.

Imaging biliary lipid secretion in the rat: ultrastructural evidence for vesiculation of the hepatocyte canalicular membrane

Physical-chemical and biological studies of hepatic bile suggest that biliary phospholipid molecules are secreted as unilamellar vesicles. Systematic ultrastructural studies of bile canaliculi were undertaken to visualize this event. Liver tissue was obtained from normal adult male rats (control), from bile salt-depleted rats (by overnight biliary diversion), and from depleted rats infused intravenously with a hydrophilic-hydrophobic congener series of common taurine-conjugated bile salts. Livers were fixed in situ either by modified chemical methods or by ultrarapid cryofixation. In control rats, chemical fixation revealed unilamellar vesicles 63 +/- 17 (+/- SD) nm in diameter, mostly free within canalicular lumena. Vesicles were infrequent in canaliculi of bile salt-depleted rats, but were present in canaliculi of rats infused with taurocholate. In cryofixed liver tissue, vesicles 67 +/- 13 nm in diameter were observed in canaliculi of control rats and bile-salt depleted rats infused with common bile salts. The majority of these vesicles were affixed to the luminal side of the canalicular membrane. The average number of vesicles per bile canaliculus was in agreement with that estimated on the basis of biliary phospholipid secretion rates, mean vesicle size, and area of close-packed phosphatidylcholine molecules. By immunoelectron microscopy, canalicular vesicles were free of actin and of a 100 kDa canalicular membrane protein. We conclude that biliary phospholipid molecules are secreted from hepatocytes into bile canalicular lumena as unilamellar vesicles approximately 63-67 nm in average diameter. We postulate that this secretion mechanism involves lumenal bile salt-induced vesiculation of lipid microdomains in the exoplasmic hemileaflet of the canalicular membrane.

Bile acid/phosphatidylcholine interactions in mixed monomolecular layers: differences in condensation effects but not interfacial orientation between hydrophobic and hydrophilic bile acid species

Monomolecular layers of undissociated bile acids and membrane lipids at the air/water interface serve as useful models for the interactions between fully ionized bile salts and physiological membranes. Employing an automated Langmuir-Pockels surface balance, surface pressures and dipole moments were measured as functions of molecular area for six dihydroxy bile acids: ursodeoxycholic acid (UDCA), deoxycholic acid (DCA), chenodeoxycholic acid (CDCA), hyodeoxycholic acid (HDCA), allodeoxycholic acid (alloDCA), and 7 alpha,12 alpha-dihydroxycholanoic acid (7,12-OH-DCA), individually and in mixed mono-molecular layers with 1-palmitoyl-2-oleoyl-sn-3-glycerophosphatidylcholine (POPC) with and without cholesterol on a 5 M NaCl subphase at pH 2. In general, monolayers of hydrophilic bile acids (UDCA, HDCA) had lower collapse pressures and surface dipole moments than hydrophobic bile acids (CDCA approximately DCA approximately alloDCA < 7,12-OH-DCA). In binary mixtures with POPC, all bile acids including the synthetic 7,12-OH-DCA and the uncommon alloDCA condensed mixed monomolecular layers, with the degree of condensation correlating positively with bile acid hydrophobicity. In contrast, none of the bile acids caused further condensation of condensed POPC/cholesterol monomolecular layers. Surface dipole moments of binary bile acid/POPC +/- cholesterol mixtures demonstrated strict additivity, implying that no change in molecular orientation of the interface occurred over film compositions that varied from 0 to 100% bile acid. We conclude that the long axes of the steroid nuclei of dihydroxy bile acids remain parallel to the interface in mixed bile acid/POPC +/- cholesterol monomolecular layers under all conditions. We infer that fully dissociated hydrophobic and, to a lesser extent, hydrophilic bile salts condense phospholipid monomolecular layers and membranes in a similar fashion.

Lith1, a major gene affecting cholesterol gallstone formation among inbred strains of mice

The prevalence of cholesterol gallstones differs among inbred strains of mice fed a diet containing 15% (wt/wt) dairy fat, 1% (wt/wt) cholesterol, and 0.5% (wt/wt) cholic acid. Strains C57L, SWR, and A were notable for a high prevalence of cholelithiasis; strains C57BL/6, C3H, and SJL had an intermediate prevalence; and strains SM, AKR, and DBA/2 exhibited no cholelithiasis after consuming the diet for 18 weeks. Genetic analysis of the difference in gallstone prevalence rates between strains AKR and C57L was carried out by using the AKXL recombinant inbred strain set and (AKR x C57L)F1 x AKR backcross mice. Susceptibility to gallstone formation was found to be a dominant trait determined by at least two genes. A major gene, named Lith1, mapped to mouse chromosome 2. When examined after 6 weeks on the lithogenic diet, the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.88) was downregulated as expected in the gallstone-resistant strains, AKR and SJL, but this enzyme failed to downregulate in C57L and SWR, the gallstone-susceptible strains. This suggests that regulation of the rate-limiting enzyme in cholesterol biosynthesis may be pivotal in determining the occurrence and severity of cholesterol hypersecretion and hence lithogenicity of gallbladder bile. These studies indicate that genetic factors are critical in determining gallstone formation and that the genetic resources of the mouse model may permit these factors to be identified.

Microscope laser light-scattering spectroscopy of vesicles within canaliculi of rat hepatocyte couplets

Employing microscope laser light-scattering spectroscopy, we investigated "primary" bile secretion into canalicular spaces of rat hepatocyte couplets in monolayer culture. Time-dependent scattered light intensities were fitted by bi-exponential decays. The "slow" decay was attributed to an undulating canalicular membrane motion, whereas the "fast" decay was consistent with rapidly diffusing intracanalicular vesicles with mean hydrodynamic radii (+/- SD) of 479 +/- 53 A. After addition of micromolar concentrations of common bile salts, increases in the amplitude of the fast component facilitated a quantitative estimate of vesicle secretion rates. A dose-response relationship with 0.1-200 microM sodium taurocholate was characterized by an initial concentration-dependent increase and then a decrease in the amplitude of the fast canalicular component. Since these taurocholate concentrations are nontoxic to cultured hepatocytes, the maximum in vesicle-sized particles at 10 microM taurocholate suggested that its critical micellar concentration of approximately 5 mM was attained within the canalicular spaces. Sodium taurolithocholate resulted in time- and dose-dependent diminution in vesicle secretion rates, which after 2 h was followed by spontaneous canalicular recovery. This suggested that acute bicellular "cholestasis" was followed by oxidative metabolism and detoxification of the monohydroxy bile salt. Microscope laser light-scattering spectroscopy should facilitate further physical-chemical and pathophysiological studies of bile secretion at the cellular level.

Update on physical state of bile

Because of recent assertions by a group of investigators that structures called "lamellae" instead of mixed micelles are present in human bile, the nature of biliary cholesterol solubilization and transport ("carriage") has again become a matter of dispute. "Lamellae" are rod- or tubular- shaped banded images observed when biles are negatively stained and dehydrated during electron microscopy; they are believed to be composed principally of biliary phospholipid (which is mostly lecithin) and cholesterol. It is well known that when mixed together in aqueous systems, lecithin and cholesterol, which are otherwise insoluble amphiphilic lipids, swell to form stacked or multilamellar liquid crystals that have regular periodicity because of the bilayer arrangement of the molecules. Provided super-micellar concentrations of cholesterol are present, multilamellar vesicles occur spontaneously in concentrated model biles, and are a frequent occurrence in human gallbladder biles that are beginning to nucleate cholesterol crystals. When multilamellar vesicles are negatively stained and dehydrated, they produce "lamellae" images by electron microscopy. Coincidentally, images of "lamellae" are also produced when purely micellar bile, either model or native is treated similarly. In this review we show that these images are an artifact. This artifact is produced by the dehydration process itself and is due to a phase change i.e. a change in molecular packing which is predicted by the appropriate phase diagram. As a consequence, a dehydrated "lamellae" phase results and the overall effect is an electron microscopic image that is identical to those produced by multilamellar vesicles in supersaturated or lithogenic biles.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium affinity for biliary lipid aggregates in model biles: complementary importance of bile salts and lecithin

BACKGROUND/AIMS

Despite putative roles of calcium in biliary physiology and gallstone formation, quantitative aspects of calcium binding to bile salt (BS) monomers, simple micelles, mixed micelles, and vesicles, which constitute the lipid aggregates in bile, remain unexplored.

METHODS

Calcium activity was measured using the calcium electrode in pathophysiologically relevant model biles composed of either individual BS species or a physiological mixture of glycine and taurine conjugates, as functions of lecithin and cholesterol contents and total lipid concentration.

RESULTS

Calcium binding increased with increasing BS concentrations and lecithin contents and varied with species (dihydroxy > trihydroxy BS) and with conjugation (unconjugated > glycine conjugates > taurine conjugates). Although lecithin/cholesterol vesicles did not bind detectable calcium, when taurocholate was incorporated into membrane bilayers, calcium binding was substantially greater than with equimolar BS alone. Added cholesterol did not alter calcium binding, despite cholesterol saturation of biliary lipid aggregates and induction of liquid crystalline and solid crystalline-phase transitions.

CONCLUSIONS

In model biles, most calcium is bound to mixed micelles, with minor contributions by BS monomers, simple micelles, and vesicles. It is proposed that BS-induced binding of calcium to vesicles and mixed micelles may be important in nucleation of cholesterol and bilirubinates from native bile.

In vitro evidence that phospholipid secretion into bile may be coordinated intracellularly by the combined actions of bile salts and the specific phosphatidylcholine transfer protein of liver

Using model systems, we explored a potential function of hepatic phosphatidylcholine transfer protein to extract biliary-type phosphatidylcholines from intracellular membranes (e.g., smooth endoplasmic reticulum) and deliver them to canalicular plasma membranes where biliary secretion occurs. We measured transfer rates of parinaroyl phosphatidylcholine, a naturally fluorescent phospholipid, from small unilamellar vesicles composed of sn-1 palmitoyl, sn-2 parinaroyl phosphatidylcholine, and egg yolk phosphatidylcholine (molar ratio 75:25) wherein the fluorophore is self-quenched to small unilamellar vesicles composed of phosphatidylethanolamine, sphingomyelin, phosphatidylserine, phosphatidylinositol, and cholesterol (molar ratios 22:22:10:8:38) representing model microsomal and canalicular plasma membranes, respectively. Following addition of phosphatidylcholine transfer protein (purified from bovine liver), fluorescence intensity increased exponentially indicating net phosphatidylcholine transfer from donor to acceptor vesicles. Submicellar concentrations of a wide hydrophobicity range of common and uncommon taurine and glycine conjugated bile salts species (anionic steroid detergent-like molecules), sodium taurofusidate (a conjugated fungal bile salt analog), and sodium dodecyl sulfate and octylglucoside, anionic and nonionic straight chain detergents, respectively, markedly stimulated phosphatidylcholine transfer protein activity. This 40-115-fold effect was most pronounced for the common bile salts and correlated positively with bile salt hydrophobicity. Thermodynamic analysis of net transfer revealed that the rate-limiting step was extraction of phosphatidylcholine molecules from donor vesicles and that bile salts facilitated their capture by enhancing both phosphatidylcholine transfer protein binding as well as perturbing phospholipid packing in vesicle bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipid molecular species influence crystal habits and transition sequences of metastable intermediates during cholesterol crystallization from bile salt-rich model bile

Despite its importance in cholesterol gallstone formation, crystallization of cholesterol from bile is poorly understood, especially with respect to the influences of other biliary lipids. We reported recently (Konikoff et al. J. Clin. Invest. 1992. 90: 1155-1160) that cholesterol can crystallize from model and native biles as filamentous crystals covered by a surface layer of lecithin molecules. During growth, filamentous crystals transformed via metastable intermediates into classical plate-like cholesterol monohydrate crystals. Using the same dilute (1.2 g/dl total lipid) bile salt-rich (97.5 moles %) model bile supersaturated with cholesterol, we have studied the effects of natural egg yolk, soy bean as well as single molecular species of lecithins, other phospholipids and related lipid classes on early filamentous cholesterol crystallization, as well as transformations between crystal habits and their growth to equilibrium cholesterol monohydrate plates. After extraction and derivatization, HPLC analysis revealed that the surfaces of filamentous crystals were enriched preferentially with lecithin molecular species having longer and more saturated sn-1 and sn-2 acyl chains compared to mixed lecithin species of whole bile. In contrast, the molecular species distribution of lecithins on equilibrium plate-like crystals resembled that of whole bile. After incubation of pre-formed anhydrous cholesterol and cholesterol monohydrate crystals in cholesterol-free lipid solutions, we demonstrated that surface-adsorbed lecithins were not preferentially enriched excluding nonspecific lecithin adsorption. Time-sequences and transformations between metastable crystalline intermediates were altered markedly by specific phospholipid species: model biles composed of saturated short-chain, medium-chain, and polyunsaturated long-chain lecithins induced rapid precipitation of short filamentous crystals that became plate-like slowly by an "arborization pattern." Long-chain saturated lecithins and natural sphingomyelins retarded cholesterol crystallization markedly, and filamentous as well as metastable intermediate crystals made transient appearances only after plate-like crystals had formed. These observations suggest that phospholipid molecular species and class influence the earliest events in cholesterol crystallization from bile salt-rich model bile. Furthermore, as the molecular species of lecithins adsorbed onto filamentous cholesterol crystals were more saturated than in whole bile and essentially identical to those in biliary vesicles, this finding provides chemical evidence for a vesicular origin of the critical cholesterol nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholesterol carriers in human bile: are "lamellae" involved?

Cholesterol, a highly insoluble molecule, is transported in bile by specialized lipid aggregates. On the basis of extensive correlations between laboratory-prepared model biles and surgically harvested native biles, it has become generally accepted that biliary cholesterol is solubilized by simple and mixed micelles, single bilayered (unilamellar) vesicles and, under certain conditions, multilamellar vesicles (liposomes or liquid crystals) all composed of bile salts, lecithin and cholesterol in different proportions. Current concepts suggest that in lithogenic biles multilamellar vesicles result from aggregation and fusion of unilamellar vesicles and are a principal source from which cholesterol precipitates to form gallstones. Recent reports now challenge the prevailing paradigm by proposing that the principal cholesterol-carrying particles in human biles are not micelles but are "lamellae" composed of stacked membrane-like bilayers of lipids. In this article, we provide a critical overview of the experiments that led to the established views of biliary cholesterol transport and to the newer lamellae hypothesis. The principal evidence for lamellae stems from negative-stain electron microscopy, an artifactprone technique when used to study lipid-rich fluids such as bile. We show that lamellar structures represent both the electron microscopic analog of multilamellar vesicles in supersaturated biles that presage the nucleation of cholesterol crystals and an electron microscopic artifact of fossilized mixed micelles that are in fact very tiny (2 to 4 nm in radius) by state-of-the-art noninvasive techniques. We argue further that the lamellae nomenclature improperly equates two fundamentally distinct physical-chemical mechanisms for cholesterol solubilization and dispersion in bile on the basis of identically appearing electron microscopic images.

Ursodeoxycholic acid in the Ursidae: biliary bile acids of bears, pandas, and related carnivores

The biliary bile acid composition of gallbladder bile obtained from six species of bears (Ursidae), the Giant panda, the Red panda, and 11 related carnivores were determined by reversed phase liquid chromatography and gas chromatography-mass spectrometry. Bile acids were conjugated solely with taurine (in N-acyl linkage) in all species. Ursodeoxycholic acid (3 alpha, 7 beta-dihydroxy-5 beta-cholan-24-oic acid) was present in all Ursidae, averaging 1-39% of biliary bile acids depending on the species; it was not detected or present as a trace constituent (< 0.5%) in all other species, including the Giant panda. Ursodeoxycholic acid was present in 73 of 75 American Black bears, and its proportion averaged 34% (range 0-62%). Ursodeoxycholic acid averaged 17% of biliary bile acids in the Polar bear (n = 4) and 18% in the Brown bear (n = 6). Lower proportions (1-8%) were present in the Sun bear (n = 2), Ceylon Sloth bear (n = 1), and the Spectacled bear (n = 1). Bile of all species contained taurine-conjugated chenodeoxycholic acid and cholic acid. In some related carnivores, deoxycholic acid, the 7-dehydroxylation product of cholic acid, was also present. To determine whether the 7 beta hydroxy group of ursodeoxycholic acid was formed by hepatic or bacterial enzymes, bile acids were determined in hepatic bile obtained from bears with chronic biliary fistulae. Fistula bile samples contained ursodeoxycholic acid, chenodeoxycholic acid, and a trace amount of cholic acid, all as taurine conjugates, indicating that ursodeoxycholic acid is a primary bile acid formed in the liver in Ursidae.(ABSTRACT TRUNCATED AT 250 WORDS)

Pan-sulfation of bile salts markedly increases hydrophilicity and essentially abolishes self- and hetero-association with lecithin

In chronic liver disease, partially and to a lesser extent completely (pan-)sulfated common bile salts are synthesized, yet little information is available concerning their physical-chemical characteristics. We studied solution properties of pan-sulfated common free, taurine and glycine-conjugated bile salts, and the interactions of taurodeoxycholate di-sulfate (TDC-S) with lecithin. By reverse-phase HPLC, pan-sulfated glycine and taurine-conjugated bile salts were very hydrophilic, with hydrophobic indices 1.7 to 2.5 units lower than their non-sulfated congeners. In contrast to non-sulfated species, pan-sulfated free and glycine-conjugated bile salts produced simple potentiometric titration curves without precipitation of bile salt below the pK'A of the carboxylic acids. By quasi-elastic light scattering, critical micellar concentrations of TDC-S fell from 28 mM in 0.15 M NaCl to 3 mM in 4.0 M NaCl, a value slightly higher than that of TDC. TDC-S formed very small micelles (hydrodynamic radii approx. 11A) that, in contrast to TDC, did not grow with increases in bile salt (7-66 mM) or NaCl (0.15-2.0 M) concentrations. TDC-S formed mixed micelles with lecithin in 0.15 M NaCl, but with a micellar zone drastically reduced compared with that of the non-sulfated congener. However, in 4 M NaCl, the micellar zone of TDC-S expanded and approached that of the non-sulfated parent compound. Therefore, under physiological conditions, pan-sulfation of common bile salts should largely eliminate their capacity to form mixed micelles with membrane lipids.