Effect of human biliary immunoglobulins on the nucleation of cholesterol

Transgenic overexpression of Niemann-Pick C2 protein promotes cholesterol gallstone formation in mice

BACKGROUND & AIMS

Niemann-Pick C2 (NPC2) is a lysosomal protein involved in the egress of low-density lipoprotein-derived cholesterol from lysosomes to other intracellular compartments. NPC2 has been detected in several tissues and is also secreted from the liver into bile. We have previously shown that NPC2-deficient mice fed a lithogenic diet showed reduced biliary cholesterol secretion as well as cholesterol crystal and gallstone formation. This study aimed to investigate the consequences of NPC2 hepatic overexpression on liver cholesterol metabolism, biliary lipid secretion, gallstone formation and the effect of NPC2 on cholesterol crystallization in model bile.

METHODS

We generated NPC2 transgenic mice (Npc2.Tg) and fed them either chow or lithogenic diets. We studied liver cholesterol metabolism, biliary lipid secretion, bile acid composition and gallstone formation. We performed cholesterol crystallization studies in model bile using a recombinant NPC2 protein.

RESULTS

No differences were observed in biliary cholesterol content or secretion between wild-type and Npc2.Tg mice fed the chow or lithogenic diets. Interestingly, Npc2.Tg mice showed an increased susceptibility to the lithogenic diet, developing more cholesterol gallstones at early times, but did not show differences in the bile acid hydrophobicity and gallbladder cholesterol saturation indices compared to wild-type mice. Finally, recombinant NPC2 decreased nucleation time in model bile.

CONCLUSIONS

These results suggest that NPC2 promotes cholesterol gallstone formation by decreasing the cholesterol nucleation time, indicating a pro-nucleating function of NPC2 in bile.

Gender-specific association of the interleukin 18 gene with symptomatic gallstone disease

BACKGROUND AND AIM

Symptomatic gallstone disease (SGSD) induced several inflammatory responses and affected extrahepatic bile ducts. Although the pathology and environmental risk factors of gallstone disease are well documented, immune or inflammatory responses in SGSD development are still inconclusive. Interleukin 18 (IL18) is a pro-inflammatory cytokine that plays an important role in immune, infectious, and inflammatory diseases because of the induction of interferon-γ. In this study, we investigated whether polymorphisms of the IL18 gene were associated with SGSD susceptibility.

METHODS

Genomic DNA was isolated from the whole blood samples of 445 patients with SGSD and 1121 gallstone-free controls. The IL18 rs549908T>G, rs5744247C>G, rs187238G>C, rs1946518T>G, and rs360719A>G polymorphisms were genotyped using predeveloped TaqMan allelic discrimination assay.

RESULTS

We found IL18 rs5744247G allele conferred protection against SGSD in female patients (odds ratio = 0.75, corrected P-value = 0.015). Haplotype analysis revealed that TGGTA protected females from SGSD development (odds ratio = 0.75, corrected P-value = 0.02).

CONCLUSIONS

Based on our findings, IL18 rs5744247C>G polymorphism could be a potential genetic marker to predict SGSD susceptibility in Han Chinese women.

Comparison of free-solution and surface-immobilized molecular interactions using a single platform

While it is generally accepted that surface immobilization affects the binding properties of proteins, it has been difficult to quantify these effects due to the lack of technology capable of making affinity measurements with species tethered and in free solution on a single platform. Further, quantifying the interaction of binding pairs with widely differing masses has also been challenging, particularly when it is desirable to tether the high molecular weight protein. Here we describe the use of backscattering interferometry (BSI) to quantify the binding affinity of mannose and glucose to concanavalin A (ConA), a 106 KDa homotetramer protein, in free solution using picomoles of the protein. Using the same platform, BSI, we then studied the effect on the binding constants of the ConA-carbohydrate interactions upon chemically immobilizing ConA on the sensor surface. By varying the distances (0, 7.17, and 20.35 nm) of the ConA tether and comparing these results to the free-solution measurements, it has been possible to quantify the effect that protein immobilization has on binding. Our results indicate that the apparent binding affinity of the sugar-lectin pair increases as the distance between ConA and the surface decreases. These observations could lend insight as to why the affinity values reported in the literature sometimes vary significantly from one measurement technique to another.

Osteopontin plays an anti-nucleation role in cholesterol gallstone formation

AIM

  To investigate the role of osteopontin in cholesterol gallstone formation.

METHODS

  Nucleation time was determined in model and human gallbladder bile in vitro. Effect of osteopontin on vesicles of bile was investigated via transmission electron microscopy. The mRNA and protein expression of osteopontin were detected in human calculus and normal gallbladder tissues, and then lipid compositions of human bile were determined via commercial kits.

RESULTS

  Osteopontin could prolong the nucleation time in a dose-dependent manner, and inhibit the pro-nucleation effect induced by calcium ion. Cholesterol crystal growth was inhibited by osteopontin in a dose-dependent manner in model and human gallbladder bile, but not affected by calcium. Furthermore, the formation, aggregation and fusion of vesicles were suppressed by osteopontin in model and human bile as demonstrated by transmission electron microscopy. The mRNA and protein expression of osteopontin in calculus gallbladder tissues were lower than those in normal tissues. The concentrations of cholesterol, phospholipid and bile acids, and cholesterol saturated index were higher and the contents of osteopontin and calcium, nevertheless, were found to be lower in lithogenic bile than those in normal controls.

CONCLUSION

  These findings indicated that osteopontin could inhibit the cholesterol gallstone formation as an anti-nucleation factor, which may be involved in the pathogenesis of cholesterol gallstone formation.

Investigation of cytotoxic T-lymphocyte-associated protein 4 gene polymorphisms in symptomatic gallstone disease

Gallstone disease (GSD), which is increasingly prevalent in Taiwan, develops through a complex process involving genetic, environmental, and immune factors. Cytotoxic T-lymphocyte-associated protein 4 (CTLA4) limits T-cell proliferation. The present study looked for associations between symptomatic GSD and polymorphisms of the CTLA4 gene. For this case-control cross-sectional study among Taiwanese, 275 patients with symptomatic GSD and 852 controls were enrolled. Genotyping of CTLA4-318 C/T, +49 A/G, and CT60 A/G single nucleotide polymorphisms (SNPs) was performed by polymerase chain reaction-restriction fragment length polymorphism. The genotype, allele, carrier, and haplotype frequencies were calculated by direct counting or with Haploview 4.1 software. Genotype, allele, carrier, and haplotype frequencies of the CTLA4 SNPs studied were equally distributed in symptomatic GSD patients and controls. No significant associations between symptomatic GSD and these 3 SNPs were observed. Our data suggest that CTLA4-318 C/T, +49 A/G, and CT60 A/G SNPs do not confer increased susceptibility to symptomatic GSD.

Roles of infection, inflammation, and the immune system in cholesterol gallstone formation

Cholesterol gallstone formation is a complex process mediated by genetic and environmental factors. Until recently, the role of the immune system in the pathogenesis of cholesterol gallstones was not considered a valid topic of research interest. This review collates and interprets an extensive body of basic literature, some of which is not customarily considered to be related to cholelithogenesis, describing the multiple facets of the immune system that appear to be involved in cholesterol cholelithogenesis. A thorough understanding of the immune interactions with biliary lipids and cholecystocytes should modify current views of the pathogenesis of cholesterol gallstones, promote further research on the pathways involved, and lead to novel diagnostic tools, treatments, and preventive measures.

Two and three-dimensional pattern recognition of organized surfaces by specific antibodies

Understanding molecular recognition of supramolecules for solid substrates is essential for designing chemical sensors and molecular devices. The rules of molecular recognition are well established at the level of single molecules. However, during the transition from molecular-scale devices to macroscopic devices, issues concerning control over recognition that are well-established at the molecular level become much more complex. Hopefully, the conceptual and practical considerations reported here will clarify some of these issues. The immune system uses antibodies to identify molecular surfaces through molecular recognition. Antibodies are thus appropriate tools to study the rules of macromolecule-surface interactions, and this was done using crystal surfaces as substrates. Crystals can be formed or introduced into organisms and should be thus treated by the organism as any other intruder, by eliciting antibodies specific to their surfaces. A structure-recognizing antibody is defined here as complementary to a certain ordered supramolecular organization. It can be considered as a mold bearing in its binding site memory of the organization against which it was elicited. On the surface of a crystal composed of relatively small organic molecules, an antibody binding site would encompass an array of 10-20 molecular moieties. The antibody binding site would not detect one molecule, but rather a two- or three-dimensional molecular arrangement on the surface, similar to a macromolecular surface. The complementarity between antibody binding site and surface is supported by stereoselective supramolecular interactions to the repetitive structural motifs that are exposed at the surface. A procedure was developed in order to isolate monoclonal antibodies that specifically recognize a certain crystalline surface. The procedure was applied in particular to crystals of cholesterol monohydrate, of 1,4-dinitrobenzene, and of the tripeptide (S)leucine-(S)leucine-(S)tyrosine (LLY). A series of antibodies were selected and studied, three of which provided reliable specific antibody-antigen structural models. The three docking models show an astounding geometrical and chemical match of the antibody binding sites on the respective crystal surfaces. We also showed that antibodies are intrinsically capable of recognition at the length scale necessary for detection of chirality. Once the structural parameters determining the antibody specificity to the target surfaces are characterized, the antibodies may be conceivably used as reporters of the existence and location of target domains with similar structure in biological milieus. In this context, we developed and characterized monoclonal antibodies specific to crystalline mixed monolayers of cholesterol and ceramide, fundamental building blocks of lipid microdomains in cellular membranes. When used on cells, one antibody indeed labels cell membrane domains composed of cholesterol and ceramide. The fundamental contribution of the approach developed here may be in the antibody ability to report on the structural organization of paracrystalline domains that cannot be determined by other means. Alternatively, structure-recognizing antibodies may be conceivably used to carry information or build connections to specific targets, which may offer interesting developments in medicine or electronics.

T-cell function is critical for murine cholesterol gallstone formation

BACKGROUND & AIMS

The formation of cholesterol gallstones is a complex process involving contributions from genes and environmental factors. Although gallbladder inflammation is believed to be common during cholelithogenesis, the role of immunologic factors is unknown.

METHODS

The role of adaptive immunity in cholesterol cholelithogenesis was analyzed utilizing immunocompetent Helicobacter spp.-infected and -uninfected BALB/c and congenic immunodeficient Rag2(-/-) (Rag) mice. Lymphocyte transfer studies were performed to determine which cellular subset was responsible for cholesterol gallstone formation. Also, gallbladder inflammation was quantified to determine the nature of the inflammatory response associated with cholelilithogenesis.

RESULTS

When fed a lithogenic diet for 8 weeks, wild-type mice developed significantly more cholesterol gallstones (27%-80% prevalence) than Rag mice ( approximately 5%, P < .05). Helicobacter spp.-infected BALB/cJ mice displayed statistically significant increases in cholesterol gallstone prevalence compared with uninfected mice (81% vs. 39%; P < .05). Transfer of splenocytes or T lymphocytes to Rag2(-/-) mice increased stone prevalence markedly (26% and 40% respectively; P < .05), whereas transfer of B cells was not appreciably cholelithogenic (13%). The adaptive immune response increased the expression of gallbladder Muc genes and accumulation of mucin gel. In addition, T cells and cholesterol monohydrate crystals induced proinflammatory gene expression in the gallbladder, which likely contributes to gallbladder dysfunction.

CONCLUSIONS

These studies indicate that T cells are critical in murine cholesterol cholelithogenesis. Furthermore, cholesterol monohydrate crystals induce expression of proinflammatory cytokines in a T-cell-dependent fashion. Acquired immunity and inflammation are likely to be crucial factors in cholesterol gallstone pathogenesis, rather then merely the result of cholelithogenesis.

Purification and characterization of 33.5 kDa vesicular protein in human bile

AIM: The present study was undertaken to purify and partially characterize the 33.5-kilodalton (33.5 kDa) vesicular protein in human bile and to explore the possible molecular mechanisms of the initial crystal nucleation process.

METHODS: The 33.5 kDa vesicular protein was isolated by ultracentrifugation and further purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions. The purified 33.5 kDa vesicular protein was subjected to N-terminal amino acid sequencing and amino acid analysis. Cholesterol crystallization activity was detected by cholesterol crystal growth assay. The sugar chain of the 33.5 kDa vesicular protein was analyzed by dot-immunobinding assay of lectin coupled to a peroxidase (HRP-DSA, HRP-ConA, HRP-WGA) and was deglycosylated using two different enzymatic approaches (N-deglycosylation and O-deglycosylation) to determine the molecular weight of the protein component, the type of linkage between polypeptide and carbohydrate components.

RESULTS: The 33.5 kDa vesicular protein with complicated glycan was an extensively glycosylated (37.3%) monomer and these sugar chains strongly bound to DSA, but did not bind to ConA. Amino acid sequencing indicated that the protein was unique. The 33.5 kDa vesicular protein exhibited potent cholesterol crystallization promoting activity in vitro with derived crystal growth curve indices It, Ig, Ic presented as 0.57, 1.52, and 1.63 respectively. Both enzymatic proteolysis and N-deglycosylation of the protein removed all activity.

CONCLUSION: These data suggest the 33.5 kDa vesicular protein may be responsible for the pathogenesis of cholesterol gallstone disease, and the sugar chains play an important role in pro-nucleating process.

Ouabain as a mammalian hormone

Endogenous ouabain changes rapidly in humans and dogs upon physical exercise and is under the control of epinephrine and angiotensin II. Hence, the steroid acts as a rapidly acting hormone. A search for a specific binding globulin for cardiac glycosides in bovine plasma resulted in the identification of the d allotype of the micro chain of IgM whose hydrophobic surfaces interact with cardiotonic steroids and cholesterol. Such IgM complexes might be involved in the hepatic elimination of cardiotonic steroids. Thus, differences in the signaling cascade starting at Na(+),K(+)-ATPase must explain any differences in the action of ouabain and digoxin in the genesis of arterial hypertension.

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.

Cholesterol crystal binding of biliary immuno-globulin A: visualization by fluorescence light microscopy

AIM: To assess potential contributions of biliary IgA for crystal agglomeration into gallstones, we visualized cholesterol crystal binding of biliary IgA.

METHODS: Crystal binding biliary proteins were extracted from human gallbladder bile using lectin affinity chromatography. Biliary IgA was isolated from the bound protein fraction by immunoaffinity chromatography. Pure cholesterol monohydrate crystals were incubated with biliary IgA and fluoresceine isothiocyanate (FITC)conjugated anti IgA at 37 °C. Samples were examined under polarizing and fluorescence light microscopy with digital image processing.

RESULTS: Binding of biliary IgA to cholesterol monohydrate crystals could be visualized with FITC conjugated anti IgA antibodies. Peak fluorescence occurred at crystal edges and dislocations. Controls without biliary IgA or with biliary IgG showed no significant fluorescence.

CONCLUSION: Fluorescence light microscopy provided evidence for cholesterol crystal binding of biliary IgA. Cholesterol crystal binding proteins like IgA might be important mediators of crystal agglomeration and growth of cholesterol gallstones by modifying the evolving crystal structures in vivo.

Genetic determinants of variation in gallbladder disease in the Mexican-American population

Since there have not been any studies that quantify the influence of genetic factors on gallbladder disease (GBD) in humans using information from families, we utilized pedigree data to explore the genetic control of variation in liability to GBD. Using an extension of a variance components approach, we performed genetic analyses of GBD using information from 32 low-income Mexican-American families with two slightly different general models incorporating several sex-specific GBD risk factors. After evaluating the relative magnitudes of the covariate effects from these two models, we identified a parsimonious model including only significant predictors of GBD. According to this model, heritability for GBD was high (h2 = 0.44+/-0.18), after accounting for the significant effects of age, leptin in both sexes, total cholesterol, and HDL cholesterol in males only. We have shown quantitatively that variation in GBD is under strong genetic control. However, there are two major limitations to our findings: (1) since GBD was defined by a self-reported clinical history rather than an ultrasound examination, the prevalence of GBD could have been underestimated; and (2) since our design did not allow for shared environmental effects, our estimate of heritability may have been inflated.

The proteins and the formation of gallstones

Cholesterol supersaturation of bile requires assistance to form gallstones. Proteins have been proposed as candidates either to facilitate or hinder the formation of stones. It is assumed that the identity of these stone proteins should be revealed in order to assess their role in the process. We have used electrodialysis of crushed stones to extract the proteins followed by 2D electrophoresis and N-terminal amino acid sequencing to characterize them. The discovery of bacterial proteins in some stones adds evidence to the importance of an inflammatory process and the deconjugation of bilirubin in mixed gallstone formation.

Concanavalin A-binding cholesterol crystallization inhibiting and promoting activity in bile from patients with Crohn's disease compared to patients with ulcerative colitis

BACKGROUND/AIMS

Crohn's disease is a risk factor for gallstone formation. In contrast, patients with ulcerative colitis have an incidence of gallstone formation comparable to the general population. The reason for this difference is not known. The aim of this study was to elucidate the factors controlling cholesterol crystallization in gallbladder bile of Crohn's disease and ulcerative colitis patients.

METHODS

Gallbladder bile was obtained by aspiration during bowel resections (26 Crohn's disease patients, 20 ulcerative colitis patients). Biliary lipid composition, crystal detection time and the effect of extraction of the concanavalin A-binding fraction on crystal formation were determined.

RESULTS

Cholesterol crystals were present in seven of the 26 bile samples of Crohn's disease-patients and one of the 20 ulcerative colitis patients. Four of the bile samples of Crohn's disease patients were fast nucleating. None of the 20 ulcerative colitis patients had fast nucleating bile. Lipid composition, total lipid concentration and CSI were not significantly different between the two groups. In Crohn's disease patients extraction of concanavalin A-binding fraction decreased crystallization in 10 bile samples but accelerated crystallization in one bile sample. In eight bile samples from ulcerative colitis patients crystallization increased after concanavalin A-binding fraction extraction.

CONCLUSIONS

Compared to ulcerative colitis patients, gallbladder bile of Crohn's disease patients showed increased cholesterol crystallization despite comparable lipid composition and cholesterol saturation index. This difference is caused by increased cholesterol crystallization-promoting activity. Bile from ulcerative colitis patients contains a Con A-binding factor which inhibits cholesterol crystallization.

Apolipoprotein A-I in bile inhibits cholesterol crystallization and modifies transcellular lipid transfer through cultured human gall-bladder epithelial cells

BACKGROUND

Apolipoprotein A-I (Apo A-I), conventionally purified by several steps including organic solvent-delipidation from plasma, inhibits cholesterol crystallization in bile. To observe a significant effect in vitro, however, supraphysiological concentrations above 100 microg/mL are required. For this reason, this protein has not been considered to play a physiological role in vivo. In the present study, we examined the cholesterol crystal growth-inhibiting effect of biliary Apo A-I at its physiological concentration, the modification of transcellular transfer of biliary lipids through cultured human gall-bladder epithelial cells (GBEC) by Apo A-I at its physiological concentration and the binding and secretion of Apo A-I by GBEC.

METHODS AND RESULTS

We purified biliary Apo A-I to near homogeneity using immobilized artificial membrane chromatography. At 5 microg/mL, biliary Apo A-I reduced cholesterol crystal mass by 50%, whereas plasma-derived, solvent-delipidated Apo A-I had no effect. Using an antibody-capture enzyme-linked immunosorbent assay, we found reduced Apo A-I concentrations in bile samples from gallstone patients when compared with bile samples from gallstone-free controls (medians, 2.35 and 9.4 microg/mL, respectively). In a GBEC line, Apo A-I (5 microg/mL) enhanced transfer of phospholipid and cholesterol from the mucosal to the serosal side of cell monolayers by approximately 50%. These cells appear to bind Apo A-I reversibly in a dose- and time-dependent manner, compatible with receptor-type binding. Cultured human gall-bladder epithelial cells also showed basal secretion of Apo A-I, which was greatly increased by exposure to model bile solutions.

CONCLUSIONS

Apolipoprotein A-I in bile, thus, has both a direct effect on cholesterol crystal formation and enhances lipid removal from gall-bladder bile by GBEC. This effect may be specific and receptor mediated. These observations support two separate roles for human biliary Apo A-I and suggest that this protein may be important in preventing the formation of cholesterol crystals (the initial step in gallstone formation) in supersaturated bile.

Effect of beta-cyclodextrin dietary supplementation on biliary proteins and their resulting cholesterol nucleating activity in pigs

We explored the possibility that the biliary protein fraction may support part of the variation in the nucleating activity previously measured in gallbladder biles of pigs. Eighteen gallbladder aspirates freshly obtained from three dietary groups (0, 5, or 10% beta-cyclodextrin) of six pigs were chromatographed to purify their total protein fraction. Proteins were quantified, and analysed through electrophoresis and immunoblotting or enzyme-linked immunosorbent assay for albumin, and five putative effectors of cholesterol crystallisation, mucins, immunoglobulin A, 130 kDa, apolipoprotein A-I, and anionic polypeptide fraction. Each total protein fraction was also assayed for its ability to influence cholesterol precipitation, when added to supersaturated model bile. The current data provided evidence that the cholesterol crystallisation-promoting activity of biliary proteins in model biles increased with the beta-cyclodextrin dietary content. This occurred without any significant change in the total biliary protein content, but was associated with a significant decrease in the concentration of albumin and apolipoprotein A-I, resulting in changes in the overall balance of proteins in bile. Comparison of these results with the crystallisation figures previously obtained from the corresponding native biles led us to conclude that biliary proteins might influence the outcome of the crystallisation process, namely the final crystal concentration at equilibrium, but would not systematically represent a major driving force for determining the velocity of crystal formation in native bile of pigs.

Biliary secretory immunoglobulin A is a major constituent of the new group of cholesterol crystal-binding proteins

BACKGROUND & AIMS

Recently we described a new group of lectin-bound biliary proteins that bind to cholesterol crystals, modify crystal morphology, and inhibit cholesterol crystallization. The aim of the current study was to characterize and identify individual members of this group of cholesterol crystal-binding proteins.

METHODS

Crystal-binding proteins were purified from human gallbladder bile by lectin affinity chromatography and preparative gel electrophoresis. Purified crystal-binding proteins were characterized by using cholesterol crystal-growth assays, immunoblotting, and amino acid analysis. For comparison, identified biliary proteins were isolated from gallbladder bile by lectin affinity and immunoaffinity chromatography.

RESULTS

The individual crystal-binding proteins with molecular weights of 74, 63, and 28 kilodaltons inhibited cholesterol crystallization in a dose-dependent manner (2.5-10 micrograms/mL). Immunoblotting with specific antibodies and N-terminal amino acid sequences revealed that the 74-kilodalton crystal-binding protein is the secretory component, the 63-kilodalton protein is the heavy chain, and the 28-kilodalton protein is the light chain of human secretory immunoglobulin (Ig) A. Isolated biliary IgA showed a potent inhibitory effect on cholesterol crystallization in model bile even at levels less than physiological concentrations (1-100 micrograms/mL).

CONCLUSIONS

Biliary secretory IgA is a major constituent of the previously described group of cholesterol crystal-binding proteins. Crystal-binding IgA may be an important modulator of crystal agglomeration into stones and stone growth in vivo.

Cholesterol crystallization in human gallbladder bile: relation to gallstone number, bile composition, and apolipoprotein E4 isoform

Patients with multiple cholesterol gallstones are at increased risk of recurrence after nonsurgical therapy, possibly because of fast biliary cholesterol crystallization. Serum apolipoprotein E4 (apo E4) is a risk factor for primary cholesterol gallstone formation as well as recurrence. We examined potential effects of stone number and apolipoprotein E genotype on crystallization and on various crystallization-influencing factors in gallbladder biles of 36 cholesterol stone patients (25 multiple stones: 10 carrying the epsilon4 allele). Biliary cholesterol saturation, bile salt composition or concentrations of total protein, immunoglobulin (Ig)A, IgG, alpha1-acid glycoprotein, haptoglobin, or mucin--all crystallization promoters--did not differ between multiple and solitary stone patients, apparently not explaining different speed of crystallization (crystal observation time 3.5 +/- 0.6 days vs. 12.7 +/- 2.4 days, respectively; P = .0003). In contrast, biliary aminopeptidase-N activities (2,607 +/- 592 mU/mL vs. 947 +/- 185 mU/mL; P = .04) were higher and IgM levels (179 +/- 39 vs. 65 +/- 8 mg/L; P = .09) tended to be higher in the case of multiple stones. Although patients carrying the epsilon4 allele had similar stone numbers and crystallization as patients without the epsilon4 allele, their cholesterol saturation index (CSI) was lower (1.08 +/- 0.09 vs. 1.54 +/- 0.13; P = .01), whereas total protein and bile salt concentrations tended to be higher with preferential taurine-conjugation. In conclusion, fast cholesterol crystallization is associated with multiple stones but not with apolipoprotein E4. Whereas fast crystallization may contribute to high recurrence rates after nonsurgical therapy in case of multiple gallstones, the mechanism for increased risk of gallstone formation in patients carrying the epsilon4 allele remains unknown.

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

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

Lipid-protein complexes as cholesterol pronucleating agents in human bile

Among the various substances which accelerate the formation of cholesterol crystals in cholesterol supersaturated bile are proteins obtained from the bile by affinity chromatography on con A-Sepharose. Several such con A binding proteins have been identified and shown to mediate acceleration of cholesterol crystal formation in vitro. However, the major protein fraction, which does not bind con A, has been studied rarely. Investigation of the effect of this latter bile protein fraction on cholesterol crystallization is the aim of this study. Contrary to results published to date, the con A nonbinding protein fraction exerted a higher cholesterol crystallization promoting activity than the con A binding fraction. Delipidation as well as proteolytic degradation sharply decreased the activity of both fractions. Albumin was identified as the main component of the con A nonbinding fraction. A lipid-protein complex formed from the lipid and albumin possessed a very high cholesterol crystallization promoting activity whereas albumin or the lipid alone showed much lower activity. Bivalent ions, especially Mn2+ and Ca2+, increased the promoting activity of the lipid-protein complex. Thus, albumin and other bile protein can bind noncovalently biliary lipid material and such lipid-protein complexes may act as the main cholesterol crystallization promoter in the human bile.

The role of the Concanavalin A-binding fraction in cholesterol crystallization in native human bile

BACKGROUND/AIMS

Many Concanavalin A-binding glycoproteins have been proposed to influence cholesterol crystallization in human bile. This has been studied mainly by addition of the Concanavalin A-binding fraction to model bile. The physiological relevance of the proteins in native bile is not yet known. The aim of this study was to establish the role of the Concanavalin A-binding fraction in cholesterol crystallization in native human gallbladder bile.

METHODS

To determine the effects of the removal of Concanavalin A-binding fraction, fresh human gallbladder bile was incubated with either Concanavalin A-Sepharose or Sepharose alone. Beads were sedimented and crystallization was studied in the supernatant.

RESULTS

Extraction of Concanavalin A-binding fraction decreased crystallization in fast-nucleating biles (Crystal Detection Time < or =4 days). Slow-nucleating biles were not affected. The effect could not be related to the content of known pronucleating proteins (IgA, IgM, haptoglobin, aminopeptidase N and alpha1-acid glycoprotein), since the slow-nucleating biles contained similar amounts of these proteins.

CONCLUSIONS

Although Concanavalin A-binding fraction always accelerated crystallization when added to model bile, removal of the same fraction from native bile often had no effect. We conclude that slow-nucleating biles in particular contain undetermined factors which regulate the activity of pronucleators.

Comparison of haptoglobin and apolipoprotein A-I on biliary lipid particles involved in cholesterol crystallization

Several proteins are known to modulate cholesterol crystallization. We recently demonstrated that haptoglobin has cholesterol crystallization promoting activity. However, this effect is still not well understood mechanistically. The current study examined the distribution of haptoglobin compared to apolipoprotein A-I (apo A-I) to micelles, vesicles and crystals as an initial step in providing a focus for further studies of the mechanism of cholesterol crystallization activity. Specific protein purification was accomplished by immunoaffinity chromatography. The crystallization-promoting activity of biliary haptoglobin, albumin and commercial apo A-I was measured by a photometric crystal growth assay. The distribution of micelles, vesicles and proteins in model bile was determined by Sepharose CL-6B column chromatography. Detection of the presence of test proteins in cholesterol crystals was determined using specific 125I-radiolabelled proteins. Haptoglobin (20 micrograms/mL) showed a significant crystallization promoting-activity, whereas apo A-I (30 micrograms/mL) only tended to show a slight inhibitory activity. The cholesterol crystal-bound protein in each case was found to be less than 1% of the total concentration of that protein that had been added to the model bile system. The elution profile of commercial apo A-I from a Sepharose CL-6B column was strikingly altered when it was added to model bile prior to elution. In contrast, the column elution profiles for both haptoglobin and albumin were unchanged when model bile was similarly added to the sample. Haptoglobin increased the amount of cholesterol found in the vesicular fraction when compared to apo A-I. Haptoglobin does not bind tightly to either biliary lipid particles or to cholesterol crystals but does increase the amount of cholesterol in vesicles by inducing a shift from micellar cholesterol (P = 0.046). This shift appears to explain in part its promoting effect on cholesterol crystallization.

Monoclonal antibody recognition of cholesterol monohydrate crystal faces

BACKGROUND

The immune system can elicit antibodies against a wide variety of antigens. We have proposed that crystal surfaces may also operate as antigens, binding specific antibodies. Here we exploit the crystal surfaces of cholesterol monohydrate to investigate antibody-surface recognition at the molecular level.

RESULTS

Four monoclonal antibodies were selected. Two specifically interact with cholesterol monohydrate crystals, and one with 1,4-dinitrobenzene crystals. The fourth interacts nonselectively with various solid substrates. The relative reactivities of the four antibodies to the different surfaces of cholesterol monohydrate and to other surfaces were compared. The nonspecific antibody adsorbs mainly at imperfections. Of the two specific antibodies, one shows a clear preference for one set of faces, relative to others, the second adsorbs selectively at one face of cholesterol monohydrate crystals.

CONCLUSIONS

Monoclonal antibodies can be selected that specifically bind to the crystal surfaces of cholesterol monohydrate. The binding sites of such antibodies appear to recognize a number of molecular moieties, exposed at the surface in a specific structural organization. Different antibodies recognize different structural organizations with varying degrees of selectivity. Antibody-crystal surface interactions may serve as convenient models for studies aimed at an understanding of the molecular bases of antibody recognition.

Biliary aminopeptidase-N and the cholesterol crystallisation defect in cholelithiasis

Several biliary proteins have cholesterol crystallisation promoting activity. One of these glycoproteins is aminopeptidase-N, a canalicular ectoenzyme. This study attempted to localise aminopeptidase-N along the biliary tree, to assess its concentration in a series of 98 patients subjected to abdominal surgery, 40 of them without gall stones, and to correlate its concentration with cholesterol crystal formation time of gall bladder bile. Aminopeptidase-N was isolated from purified native biliary vesicles. A specific polyclonal rabbit anti-aminopeptidase-N antibody was prepared for quantitative immunoblotting and for immunolocalisation. Tissue was obtained from liver biopsy specimens and from gall bladders removed at surgery because of gall stone disease. Aminopeptidase-N was immunolocalised to the apical membranes of hepatocytes and to the apical pole of ductular and gall bladder mucosal cells. The nucleation time of gall bladder bile was mean (SD) 4 (3) days in the gall stone group, compared with 21 (18) days in the control group (p < 0.001). Total absolute biliary protein and aminopeptidase-N concentrations were similar in both the control and gall stone patients. There was a reciprocal significant correlation, however, between the nucleation time and the relative aminopeptidase-N concentration (r = -0.35, p < 0.01) only in the gall stone group of patients. This study shows that this apical transmembrane ectoenzyme with cholesterol crystallisation promoting activity is present along the biliary tree and the hepatocyte. These findings support the concept that high concentrations or qualitative changes of biliary aminopeptidase-N contribute to cholesterol gall stone formation.

Biliary alpha 1-acid glycoprotein concentrations in gallstone-free controls and in patients with multiple or solitary cholesterol gallstones

We recently identified a promoting glycoprotein in the concanavalin A-bound fraction of gallbladder bile as a biliary form of alpha 1-glycoprotein (AAG). The concentration of biliary AAG appears to exert an important promoting effect on the speed of cholesterol nucleation in many patients with cholesterol gallstone disease. In the current study, we provide information about the biliary concentration of AAG as well as the amount and comparative potency of its subfractions in patients with and without cholesterol gallstone disease. The amount of total biliary AAG and the amounts of its different isoforms separated by concanavalin A affinity chromatography were measured by ELISA. Estimates of absolute concentrations of AAG for each sample were normalized to the sample total protein content to give relative AAG values. The promoting activity (potency) of immunopurified biliary AAG from gallstone patients and gallstone-free controls on cholesterol crystallization was compared by a crystal growth assay. The mean absolute concentration of AAG in gallstone-free controls was not significantly different from multiple stone patients. The relative concentration of AAG (micrograms per milligram total protein) was significantly increased in patients with multiple stones when compared to controls (P < 0.05), and both the absolute and relative concentrations of AAG (micrograms per milligram bile), were three- and to five fold higher in a number of these patients. The functional activity and distribution of AAG in different subfractions was similar in gallstone patients and gallstone-free controls. The relative concentration of biliary AAG is significantly greater in cholesterol gallstone patients with multiple stones than in gallstone-free controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between biliary alpha 1-acid glycoprotein concentration and cholesterol crystal nucleation time in gallstone disease

A biliary form of the alpha 1-acid glycoprotein (AAG) promotes cholesterol crystallization in the lower-molecular-weight, concanavalin A-bound fraction of gallbladder bile. In addition, bile AAG concentration is higher in cholesterol gallstone patients with multiple stones than in control patients without gallstone disease. In this study we sought to determine whether the increased biliary concentration of AAG in cholesterol gallstone patients is accompanied by a more rapid nucleation time in patients with multiple stones. AAG concentration in native biles was measured by ELISA. Nucleation time was measured using a standard microscopy method. The concentration of biliary AAG was then related to nucleation time in biles from the same patients. Nucleation times were significantly shorter (< or = 5 days) in cholesterol gallstone patients with raised AAG concentrations (P < 0.03). There was a significant (P = 0.004) negative correlation (r = -0.53) between nucleation time and the AAG concentration in cholesterol gallstone patients with multiple stones. The concentration of biliary AAG appears to exert an important influence on the speed of cholesterol nucleation in bile in many patients with cholesterol gallstone disease.

Specificity in the recognition of crystals by antibodies

We show that IgG molecules isolated from the serum of rabbits injected with crystals of monosodium urate monohydrate, magnesium urate octahydrate and allopurinol, can each catalyze the nucleation of the same type of crystal to which they were exposed. These results generalize previous findings related to monosodium urate monohydrate and assess the idea that the invasion of a foreign crystal into an organism may amplify a population of antibodies which bear in their binding sites an imprint of the crystal surface structure. Such antibodies can further act as nucleating templates which accelerate crystal formation in vitro. Antibodies isolated from rabbits injected with sodium urate crystals do not cross-react or cross-react only to a low extent with antibodies isolated from rabbits injected with crystals of either magnesium urate or allopurinol. These results indicate a high specificity of the elicited antibodies, as these can distinguish between nuclei of crystals having similar molecular and structural characteristics.

Diffusion of substances into human cholesterol gallstones

BACKGROUND/AIMS

The possibility that substances penetrate gallstones and accumulate after stones have formed has not been examined. The specific aims of this study were to determine whether cholesterol gallstones are permeable and, if so, the effect of molecular weight on permeability.

METHODS

Cholesterol gallstones from patients with multiple stones were collected during surgery and incubated in fluorescein solution or in solutions of fluoresceinated albumin or immunoglobulin (Ig) G. To determine egress from the stones, some stones were removed from the fluoresceinated solution after incubation and placed in bicarbonate buffer. The total area of the stone and the area of dye that had diffused into the calculi were calculated. To determine mass of penetrating IgG, stones were powdered after incubation, and IgG was measured by an enzyme-linked immunosorbent assay.

RESULTS

All substances penetrated stones. Although all compounds tested diffused back out of the stones when they were replaced in buffer, proteins did so more slowly than fluorescein.

CONCLUSIONS

Substances of different molecular weights can diffuse into and out of cholesterol gallstones. These findings must be taken into account when considering the role of substances contained in stones on stone formation and growth.

Identification of a 130-kilodalton human biliary concanavalin A binding protein as aminopeptidase N

BACKGROUND/AIMS

Human gallbladder bile contains a group of nonmucin glycoproteins that binds to the lectin concanavalin A (con A) and has been reported to promote cholesterol monohydrate crystal nucleation, an event preceding the formation of gallstones. Several of these proteins, including a 130-kilodalton protein, have been isolated and shown to promote nucleation in vitro. The aim of this study was to identify this and other major biliary con A binding glycoproteins.

METHODS

Gallbladder bile was chromatographed on con A agarose, and the eluted proteins were electrophoresed, blotted, and subjected to amino-terminal sequence analysis.

RESULTS

The major con A binding proteins were identified as aminopeptidase N (a 130-kilodalton protein), alpha 2 macroglobulin, hemopexin, immunoglobulin heavy chains, and the beta chain of haptoglobin. After further purification, aminopeptidase N was found to be enzymatically active and to promote cholesterol crystallization at its approximate physiological concentration in bile.

CONCLUSIONS

It is likely that aminopeptidase N is the previously characterized 130-kilodalton biliary crystallization promoting protein. Aminopeptidase N is probably released from the biliary canalicular membrane by the detergent activity of bile salts and may be one factor that promotes cholesterol crystallization in the gallbladder.

Cholesterol crystallization-promoting activity of aminopeptidase-N isolated from the vesicular carrier of biliary lipids

Different hydrophobic glycoproteins are associated to native biliary vesicles, which are the major carrier of biliary cholesterol. Some of these proteins promote cholesterol crystallization, a key step in cholesterol gallstone formation. This study was specifically conducted to identify the 130 kDa biliary vesicle-associated glycoprotein and to determine its in vitro effect on the cholesterol crystal formation time. The 130 kDa vesicular glycoprotein was identified as aminopeptidase-N by amino acid sequencing and specific enzymatic assay. Polyclonal antibodies raised against aminopeptidase-N allowed us to determine its concentration in human hepatic bile, which varied from 17.3 to 57.6 micrograms/ml. Aminopeptidase-N showed a concentration-dependent cholesterol crystallization activity when it was added to supersaturated model bile at a concentration range usually found in native bile. Because of this promoting effect on in vitro cholesterol crystal formation, we suggest that biliary aminopeptidase-N may play a critical role in the pathogenesis of cholesterol gallstone disease.