A calcium-binding protein in bile and gallstones

Osteopontin Expression in Patients with Hepatolith

To investigate the role of osteopontin (OPN) in the formation of hepatolith, we have immunohistochemically studied the involvement of OPN in hepatolithiasis in the intrahepatic bile ducts and the intramural and extramural glands, and in stones. In our hospital, 17 hepatic resections for hepatolithiasis were carried out from June 2006 to October 2009. Immunohistochemistry was performed on 17 liver specimens with hepatolith and on 24 control liver specimens without hepatolith. We compared the osetopontin expression in the epithelia of the intrahepatic bile ducts and peribiliary glands or periglandular macrophages. Staining for OPN was positive in the cytoplasm of the epithelial cells of stone-containing intrahepatic bile ducts and the intramural and extramural glands, and in stones. But there were no significant differences between the hepatolithiasis group and the control group for the OPN immunoreactivity of the luminal surface of the epithelial cells of the intrahepatic bile ducts and intramural and extramural glands. The stone-containing intrahepatic bile ducts were infiltrated by macrophages that showed intense staining for OPN. The core and matrix of the stones showed OPN immunoreactivity. The degree of OPN from the cytoplasm of the epithelia of the intrahepatic bile ducts and peribiliary glands or periglandular macrophages was different between hepatolithiasis and control groups. Our result suggests that the OPN from the intrahepatic bile ducts and peribiliary glands plays a role in the formation of intrahepatic stone.

An immunohistochemical study of osteopontin in hepatolithiasis

BACKGROUND/PURPOSE

Mucin glycoprotein from the intrahepatic bile ducts, and from the intramural and extramural glandular epithelia, is thought to contribute to the formation of the matrix, or nucleus, in hepatolithiasis and to contribute to biomineralization systems. Osteopontin (OPN), a noncollagenous acidic bone matrix glycoprotein that possesses calcium-binding properties. To investigate the role of OPN in the genesis of hepatolithiasis, OPN was immunohistochemically studied in the intrahepatic bile ducts, intramural and extramural glands, and in stones.

METHODS

An immunohistochemical study was performed in 21 human liver specimens with hepatolithiasis, by the EnVision (Dako Japan, Kyoto, Japan) method.

RESULTS

Staining for OPN was strongly positive in the epithelium of stone-laden intrahepatic bile ducts, intramural and extramural glands, and in stones. The stone-laden intrahepatic bile ducts were infiltrated by macrophages showing intense staining for OPN. Sections of the hepatolithiasis, viewed under low magnification, showed a lamellar pattern of OPN immunolabeling, and they showed a reticular pattern under high magnification. In the center of the stones, large nestlike membranous structures were frequently present.

CONCLUSIONS

Our result indicates that OPN, an acidic glycoprotein from intrahepatic bile ducts and from intramural and extramural glandular epithelia, seems to be involved in lithiasis, both as a core protein in the early phase, and in the late phase.

Intracellular liposomes and cholesterol deposits in chronic cholecystitis and biliary sludge

Ultrastructural study of a group of selected specimens of chronic cholecystitic gallbladders reveals cholecystocyte changes characterized by abraded and altered microvilli accompanied by mitochondrial damages in the apical regions as well as mucus accumulation with aggregated, angulated lysosomes and heterogeneous liposomes. These liposomes contain needle-like crystals, probably rich in cholesterol. Many fragments of cholecystocystes and damaged organelles or contents can be found in the biliary sludge. These data support previous reports suggesting that there is an association between cholecystitis and the presence of cholelithiasis, subsequent to the production of altered bile. The present data suggest that disintegrating, sloughed cholecystocyte contents also contribute to the bile sludge, a complex milieu enriched by lipids, cholesterol deposits, altered mucus due in part to changes in expression of apomucins. The instability of prolonged storage of such modified bile, caused and/or accompanied by other associated metabolic defects, including gallbladder sluggishness, would favor the nucleation and the enlargement of gallstones. Based on the aforementioned data, a comprehensive sequence for cholecystocyte ultrastructural alterations and pathologies is proposed, as a result of chronic cholecystitis.

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.

Physical and metabolic factors in gallstone pathogenesis

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

Gallstone formation. Local factors

Bile supersaturation is necessary for cholesterol gallstones to form. Not all people with supersaturated bile form gallstones, however, and additional factors must be present. The role of pronucleating substances has been extensively studied. Of these, proteins, especially mucin, are best understood. Mucin is secreted by the gallbladder epithelium and may act as a nidus for crystal nucleation. Other proteins that may act as pronucleators include alpha 1-acid glycoprotein, alpha 1-antichymotrypsin, phospholipase C, and a small calcium binding protein. The role of antinucleating factors is less well understood. Certain drugs, including octreotide and ceftriaxone, may also predispose to stone formation. Another local factor is gallbladder stasis, a well-known risk factor for pigment stone formation. More recent research has focused on the role of bacterial infection, which has long been believed to be a factor in pigment gallstone formation. Newer data also support a role for infection in cholesterol gallstone pathogenesis. Additionally, genetic factors that may predispose a patient to cholesterol gallstones have been identified in mice and in humans.

A peptide that inhibits hydroxyapatite growth is in an extended conformation on the crystal surface

Proteins play an important role in the biological mechanisms controlling hard tissue development, but the details of molecular recognition at inorganic crystal interfaces remain poorly characterized. We have applied a recently developed homonuclear dipolar recoupling solid-state NMR technique, dipolar recoupling with a windowless sequence (DRAWS), to directly probe the conformation of an acidic peptide adsorbed to hydroxyapatite (HAP) crystals. The phosphorylated hexapeptide, DpSpSEEK (N6, where pS denotes phosphorylated serine), was derived from the N terminus of the salivary protein statherin. Constant-composition kinetic characterization demonstrated that, like the native statherin, this peptide inhibits the growth of HAP seed crystals when preadsorbed to the crystal surface. The DRAWS technique was used to measure the internuclear distance between two 13C labels at the carbonyl positions of the adjacent phosphoserine residues. Dipolar dephasing measured at short mixing times yielded a mean separation distance of 3.2 +/- 0.1 A. Data obtained by using longer mixing times suggest a broad distribution of conformations about this average distance. Using a more complex model with discrete alpha-helical and extended conformations did not yield a better fit to the data and was not consistent with chemical shift analysis. These results suggest that the peptide is predominantly in an extended conformation rather than an alpha-helical state on the HAP surface. Solid-state NMR approaches can thus be used to determine directly the conformation of biologically relevant peptides on HAP surfaces. A better understanding of peptide and protein conformation on biomineral surfaces may provide design principles useful for the modification of orthopedic and dental implants with coatings and biological growth factors that are designed to enhance biocompatibility with surrounding tissue.

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.

Calcium and the anionic polypeptide fraction (APF) have opposing effects on cholesterol crystallization in model bile

BACKGROUND/AIMS

Cholesterol gallstones contain both calcium and biliary proteins, but their respective roles in gallstone pathogenesis are unknown. We have studied the effects of calcium and a major biliary protein, anionic polypeptide fraction, on the process of cholesterol crystallization in bile.

METHODS

Anionic polypeptide fraction was purified from human bile. Model bile composed of cholesterol, egg yolk lecithin and sodium taurocholate was prepared in a lipid concentration (18 mM, 37 mM, and 120 mM, respectively) simulating lithogenic human gallbladder bile. The crystallization process was observed by phase contrast light microscopy, and sequential separation of precipitable cholesterol structures by sucrose density gradient ultracentrifugation.

RESULTS

Addition of calcium, or anionic polypeptide fraction alone, or both together did not influence the crystal observation time of bile (the time which elapsed from initiation of supersaturation to the first appearance of crystals). However, the rate and quantity of cholesterol precipitation and crystal formation were affected by both. Calcium increased in a dose-dependent manner the cholesterol monohydrate crystal mass before apparent equilibrium was reached. This effect was inhibited by anionic polypeptide fraction, which increased the amount of cholesterol within precipitable phospholipid vesicles, and decreased the rate of crystal formation. Fluorescence-labeled anionic polypeptide fraction revealed that anionic polypeptide fraction (with and without calcium) was primarily associated with vesicle aggregates.

CONCLUSIONS

Our data demonstrate that calcium and anionic polypeptide fraction have opposing effects on the process of cholesterol crystallization and the resultant crystal mass without influencing the crystal observation time of bile. These findings suggest that biliary proteins, in addition to being crystallization effectors by themselves, may further influence cholesterol crystallization and gallstone formation by interacting with calcium and possibly other elements that coexist in bile.

Cholelithiasis induced in the Syrian hamster: evidence for an intramucinous nucleating process and down regulation of cholesterol 7 alpha-hydroxylase (CYP7) gene by medroxyprogesterone

This report reviews previously published studies from our laboratory and shows some recent morphological data obtained with scanning and transmission electron microscopy regarding gallstone formation and alteration of the gallbladder epithelium in the Syrian hamster model. Both male and female hamsters were treated with female sex steroids (estradiol alone, estradiol and medroxyprogesterone, medroxyprogesterone alone) during one month. The results show that the Syrian hamster is a good model to study bile changes, gallbladder structure changes, including gallstone formation, and the regulation of cholesterol metabolism at the molecular level. Arguments in favor of this animal model are presented and, during gallstone formation, epithelial cell changes, anionic mucus secretion, and formation of gallbladder luminal deposits can be demonstrated. Recent molecular biology observations related to the effect of female sex steroids on liver cholesterol 7 alpha-hydroxylase (CYP7) gene suggest that progestin alone or primed by estrogen down regulates CYP7 transcription and activity. In addition, progesterone in cell culture systems has been shown to enhance intracellular accumulation of free cholesterol by increasing its uptake and synthesis and by decreasing its esterification by inhibiting the activity of acylcoenzyme A: cholesterol acyltransferase. Non-esterified cholesterol is free to migrate to the extracellular spaces and may contribute to nucleation within the bile. It is suggested that these effects of progesterone on cholesterol metabolism combined with the CYP7 gene down regulation, physical changes in the mucus and the hypomotility of the gallbladder and biliary ducts result in hypersaturation of cholesterol in the bile which favors gallstone formation.

Ultrastructural aspects of human gallbladder epithelial cells in cholelithiasis: production of anionic mucus

The surface epithelium of 28 gallbladders removed during elective cholecystectomies and pathology collection was studied ultrastructurally. Focusing on 10 of the 28 cases that were diagnosed as cholecystitis, we found that the epithelium displayed numerous apical mucous granules and bulging apical apices. Mucous granule changes included 1) hyperproduction of secretory granules of neutral type containing an electron-dense proteinaceous spherule, similar to that described in other mucus-producing glands of the digestive system, and 2) production of anionic, osmiophilic secretory mucus. Other alterations of the surface epithelial cells included the production of bizarre surface appendages resembling primitive cilia without axoneme and epithelial excrescences.

Comparative morphology of the gallbladder and biliary tract in vertebrates: variation in structure, homology in function and gallstones

A review of investigations on the morphology of the gallbladder and biliary tract in fish, reptiles, amphibians, birds, and mammals was performed. Scanning electron microscopy, transmission electron microscopy, and light microscopy observations by the authors were also included. Variations in the presence or absence of a gallbladder, surface epithelium of the gallbladder, and differences in the morphology of the biliary tract in vertebrates were reported. Many differences were diet-related. Despite some dissimilarities observed, analogous functioning of the biliary system was accomplished by its various components, with the biliary ducts performing the function of the gallbladder when this organ was absent. In addition, the occurrence of peculiar parasitism and gallstones among some cases of vertebrates, including humans, was presented.

Gallstone formation and gallbladder bile composition after colectomy in dogs

A high prevalence of gallstones has been described in patients following colectomy. The aim of this study was to examine whether lithogenicity is attributed to colectomy. In the present study, changes in gallbladder bile composition and the mechanism of gallstone formation after colectomy were examined in dogs. Ten mongrel dogs underwent restorative proctocolectomy. Seven dogs which received sham operations served as controls. Over a 12-week postoperative period, samples of gallbladder bile, formed gallstones and serum were collected and analyzed. In 7 of the 10 (70%) colectomized dogs, gallstones were found in the gallbladder, while the control dogs had no stones. Macroscopically the gallstones were similar to black pigment stones observed in humans. Chemical analysis and Fourier transform-infrared spectroscopy examination revealed that the stones were composed mainly of sodium bilirubinate and proteins, with minor amounts of calcium salts and cholesterol. Significant increases in biliary pH and concentrations of ionized calcium and unconjugated bilirubin were observed in the gallbladder bile of the colectomy group compared with that of the control group. The total bile acid and total bilirubin concentrations were significantly decreased in the colectomy group. Cholesterol crystal nucleation did not occur. The inhibitory effect of gallbladder bile on calcium carbonate precipitation in an in vitro assay system was preserved even after colectomy. In conclusion, proctocolectomy increases the concentration of unconjugated bilirubin in gallbladder bile and induces pigment gallstones which are composed mainly of sodium bilirubinate and proteins since calcium ions and cholesterol are stabilized in dogs.

Regulation of protein secretion into bile: studies in mice with a disrupted mdr2 p-glycoprotein gene

BACKGROUND & AIMS

Protein is secreted into bile via several independent pathways. The aim of this study was to investigate whether these pathways are influenced by secretion of biliary lipid.

METHODS

Protein secretion and biliary lipid output were studied in wild-type mice (+/+), heterozygotes (+/-), and homozygotes (-/-) for mdr2 gene disruption. Biliary lipid and protein output were varied by infusion with taurocholate (TC) and tauroursodeoxycholate (TUDC).

RESULTS

Exocytosis and transcytosis were unaltered in (-/-) mice. Infusion with TC strongly induced secretion of alkaline phosphatase in (-/-) mice but had little effect in (+/-) and (+/+) mice. Infusion with TUDC had little effect on alkaline phosphatase output. In contrast, both TUDC and TC strongly stimulated secretion of aminopeptidase N and lysosomal enzymes in (+/+) mice but had no effect in (-/-) animals. Aminopeptidase N secretion correlated with phospholipid output, but only at high flux. At low flux, aminopeptidase N was secreted independently from both phospholipid and bile salts.

CONCLUSIONS

The canalicular membrane enzymes alkaline phosphatase and aminopeptidase N are secreted via separate pathways. Part of alkaline phosphatase output is controlled by bile salt hydrophobicity, whereas at high lipid flux, aminopeptidase N secretion seems to be coupled to phospholipid output. Lysosomal enzymes follow the latter pathway.

Interaction of bovine gallbladder mucin and calcium-binding protein: effects on calcium phosphate precipitation

BACKGROUND & AIMS

Gallstones consist of calcium salts and cholesterol crystals, arrayed on a matrix of gallbladder mucin (GBM), and regulatory proteins like calcium-binding protein (CBP). To determine if interactions between CBP and GBM follow a biomineralization scheme, their mutual binding and effects on CaHPO4 precipitation were studied.

METHODS

Binding of CBP to GBM was assessed by inhibition of the fluorescence of the complex of GBM with bis-1,8-anilinonaphthalene sulfonic acid (bis-ANS). The effects of the proteins on precipitation of CaHPO4 were assessed by nephelometry and gravimetry. Precipitates were analyzed for calcium, phosphate, and protein.

RESULTS

CBP and bis-ANS competitively displaced each other from 30 binding sites on mucin, with a 1:1 stoichiometry and similar affinity. The rate of precipitation of CaHPO4 was retarded by mucin and CBP. Precipitate mass was unaffected by GBM alone but decreased with the addition of CBP. Complexing CBP with GBM abolished or moderated this latter effect, altered precipitate morphology, and changed the stoichiometric ratios of Ca to PO4 in the precipitates from 1:1 to 3:2. Mucin and CBP were incorporated into the precipitates.

CONCLUSIONS

These studies suggest that the formation of calcium-containing gallstones is a biomineralization process regulated by both GBM and CBP.

Bilirubin inhibits calcium carbonate precipitation in gallbladder bile

BACKGROUND

Previous studies have shown that human bile contains one or more factors that inhibit the precipitation of calcium carbonate from supersaturated solutions of calcium chloride and sodium bicarbonate. Lower concentrations of this factor have been observed in the gallbladder bile of patients with calcified gallstones. We hypothesized that gallbladder bile contains factors that inhibit calcium carbonate and these factors are present in varying concentrations in normal persons and in patients with cholesterol gallstones with and without calcium carbonate.

METHODS

Gallbladder bile of patients without gallstones (n = 8) and of patients with cholesterol gallstones containing either calcium carbonate (n = 8) or other calcium salts (n = 8) was assayed for calcium carbonate inhibition. Individual components of bile (bilirubin, phospholipid, bile salts, and albumin) were tested in different concentrations in the same assay system. In addition, samples of model bile were tested.

RESULTS

An inhibitory factor for calcium carbonate precipitation was present within all human gallbladder bile, irrespective of the absence, presence, or type of gallstones. The addition of a bilirubin-albumin solution to a supersaturated solution of calcium chloride and sodium bicarbonate entirely blocked precipitation of calcium carbonate from solution. In addition, serial dilutions of bilirubin exhibited a linear response between bilirubin concentration and inhibitory effect. Model bile and phospholipid dissolved in sodium taurocholate also exhibited a modest inhibitory effect on calcium carbonate precipitation.

CONCLUSIONS

We conclude that bilirubin, cholesterol, and phospholipids incrementally interfere with calcium carbonate precipitation in supersaturated solutions through the preferential formation of a soluble calcium complex.