Epitope mapping of the human biliary amphipathic, anionic polypeptide: similarity with a calcium-binding protein isolated from gallstones and bile, and immunologic cross-reactivity with apolipoprotein A-I

Development of oligoclonal nanobodies for targeting the tumor-associated glycoprotein 72 antigen

The tumor-associated glycoprotein 72 (TAG-72) is a membrane mucin whose over-expression is correlated with advanced tumor stage and increased invasion and metastasis. In this study, we identified a panel of four nanobodies, single variable domains of dromedary heavy-chain antibodies that specifically recognize the TAG-72 antigen. All selected nanobodies were shown to selectively bind to this cancer-related molecule with low-nanomolar affinities and do not cross-react with other antigens, such as MUC1 or HER2. Furthermore, they can detect TAG-72 in concentrations as low as 5 U/ml which is valuable in sensitive detection of this molecule in cancerous patients. Cell ELISA experiments proved their ability for binding to the native target antigen on TAG-72 expressing cells while not showing any reactivity to HT-29 cells, a TAG-72-negative cell line. Using competition studies, we found that each nanobody recognizes a distinct epitope on the TAG-72 antigen that is different from the one recognized by the mouse anti-TAG-72 antibody, CC49. Considering their high specificity, reduced immunogenicity and multi-targeting behavior, these oligoclonal nanobodies represent a promising tool to target TAG-72 over-expressing tumor cells.

High density lipoprotein-anionic peptide factor effect on reverse cholesterol transport in type 2 diabetic patients with and without coronary artery disease

OBJECTIVES

To verify if HDL3 Anionic Peptide Factor (HDL3-APF) is as an apolipoprotein that promotes the reverse cholesterol transport.

DESIGN AND METHODS

We investigated a possible association between plasma HDL3-APF concentration, cholesterol efflux from Fu5AH cells and cholesteryl ester transfer protein (CETP) activity in type 2 diabetic patients with coronary artery disease (CAD) (n=36), those without CAD (n=20), and 37 healthy subjects.

RESULTS

Plasma APF concentrations were decreased in diabetics with CAD compared to controls (p<0.01). Cellular cholesterol efflux was decreased in diabetics without and with CAD, (p<0.01 and p<0.001 respectively). CETP activity was significantly elevated in all patient groups. Multiple linear regression analysis shows that cholesterol efflux was independently and positively related only to APF concentrations in controls.

CONCLUSIONS

APF is likely to be a key independent factor for promoting cellular cholesterol efflux in healthy subjects. However this association is altered in type 2 diabetes.

The anionic peptide fraction is present on the gallbladder apical epithelium and favours biliary cholesterol absorption

BACKGROUND/AIMS

We investigated (a) in vitro and in vivo the changes of biliary mass of the anionic peptide fraction, apolipoproteinA-I, immunoglobulin-A, albumin and cholesterol over time in the excluded gallbladder and (b) in vivo the localization in the gallbladder epithelium of the anionic peptide fraction and cholesterol absorbed from bile.

METHODS

Native bile was substituted with pig bile containing radiolabeled cholesterol in the in vitro isolated intra-arterially perfused pig gallbladder (n=9) and in vivo in anestethized pigs with excluded gallbladders (n=6). The amount of cholesterol (scintillation counting) and proteins (enzyme-linked immunosorbent assay) in gallbladder bile were measured over time. The localization of the anionic peptide fraction and cholesterol absorbed from bile in the gallbladder epithelium was studied in vivo by immunohistochemistry and fluoro-phospho-imager analysis.

RESULTS

The rate of biliary cholesterol disappeared from bile was a function of the initial concentration and of the biliary mass changes over time of the anionic peptide fraction, but not of that of the other biliary proteins. The anionic peptide fraction colocalized with biliary cholesterol absorbed by the gallbladder on the apical side of gallbladder epithelial cells.

CONCLUSIONS

These data indirectly suggest that biliary anionic peptide fraction could favour biliary cholesterol absorption by the gallbladder epithelium.

Anionic Peptide Factor/Phosphatidylcholine Particles Promote the Inhibition of Vascular Cell Adhesion Molecule-1 in Human Umbilical Vein Endothelial Cells

OBJECTIVE

High-density lipoproteins (HDLs) have significant cardiovascular benefits by retarding the progression of atherosclerosis. One of the mechanisms is the inhibition by HDLs of the vascular cell adhesion molecule-1 (VCAM-1) expression in endothelial cells. Our objective was to test the effect on VCAM-1 expression by the human umbilical vein endothelial cells (HUVEC) of a minor HDL2 and HDL3 apolipoprotein, the anionic peptide factor (APF). The peptide has previously been found to develop some beneficial effects against atherosclerosis, i.e. by promoting the cholesterol efflux from endothelial cells.

METHODS

We examined the effects of two HDL apolipoproteins A-I and APF, either in presence or absence of phosphatidylcholines (PCs), or free PCs, on the expression of VCAM-1 by HUVEC. The cells were stimulated with either the tumor necrosis factor-alpha (TNFalpha, 500 pg/ml) or the calcium bound to heparin (10 microg Ca2+/ml, 50 microg heparin/ml).

RESULTS

In the presence of TNFalpha, only the free PCs (0.25 and 1 mM) developed an inhibitory effect (up to 50%). In the absence of TNFalpha and in the presence of calcium bound to heparin, either the lipid-free APF (3.5 microM) or the APF/PC complexes (1:57 molar ratio) or the free PCs (0.25 mM) exhibited a substantial inhibitory effect (72, 71 and 42%, respectively).

CONCLUSION

Our present findings suggest for the first time that one of the mechanisms of the antiatherogenic action of APF involves the inhibition of VCAM-1 expression by HUVEC. The peptide, through its phospholipid-binding and its calcium antagonist abilities, appears to confer on the HDLs a protective effect against the early cellular event of the inflammatory process.

The endothelial cholesterol efflux is promoted by the high-density lipoprotein anionic peptide factor

The prevention of atherosclerosis depends on the high-density lipoprotein (HDL) capacity to stimulate the efflux of unesterified cholesterol (UC). We tested here the effects of 2 HDL apolipoproteins, apo A-I and the 7-kd anionic peptide factor (APF), on the UC efflux by human endothelial ECV 304 cells in culture. Apolipoprotein A-I (10 micromol/L) or APF (3.5 micromol/L) in lipid-free forms or small particles (13 nm with apo A-I or 19 nm with APF) were incubated in the presence of [4-14C]UC. The phosphatidylcholines (PCs) were present either at a low level (0.35 mmol/L with apo A-I or 0.20 mmol/L with APF) or at a high level (1 mmol/L with apo A-I). We also tested either large 53-nm bile lipoprotein complex-like particles (3.5 micromol/L APF [13 microg/500 microL]) with a high PC level (0.65 mmol/L) or a 9-residue synthetic peptide (13 microg/500 microL), derived from the NH2-terminal domain of HDL3-APF, in a lipid-free or low-lipidated (0.20 mmol/L PCs) form. A control was developed in absence of the added compounds. A rapid [4-14C]UC efflux mediated by APF added in free form or in 19-nm complexes was 2.2- to 2.3-fold higher than that mediated by apo A-I in free form or in 13-nm particles (P < .05). The level of this high APF-related efflux was comparable with that obtained with the 12-nm native HDLs (10 micromol/L apo A-I) or free PCs (1 mmol/L). The increase in the UC efflux was much more limited (1.4-fold) in the presence of the 53-nm APF/high-PC particles, but it was higher than that mediated by apo A-I. In addition, the efflux mediated by the synthetic peptide, in lipid-free or low-lipidated form, constituted the major part of that related to the full-length APF. Thus, all these particles are very active HDL components, able to act as cholesterol acceptors. Interestingly, we further showed a new anti-atherogenic property of APF as well as its metabolic importance and clinical relevance. By its involvement in the first step of the reverse cholesterol transport, APF could reduce the risk of cardiovascular disease.

High-fat diets impede the lowering effect of cyclosporine a on rat brain lipids and interact with the expression of apolipoproteins E and J

Cyclosporine A (CsA), a common immunosuppressive agent, produces hyperlipidemia and apolipoprotein profile alterations in plasma as well as neurological and psychiatric complications. In rats, 10 mg CsA/kg/d treatments for 3 wk induce alterations of the electroencephalogram, and of the blood and brain lipids. Using this model, we evaluated whether triacylglycerol (TG)- and lecithin (PC)-enriched diets, reported to decrease epileptic episodes (TG) and to improve memory, could modify the effects of CsA treatment on brain lipids and possibly change apolipoprotein (apo) E and apoJ gene expression. To evaluate this hypothesis, three groups of rats were treated for 3 wk with CsA and received a low-fat, PC, or TG diet. Three other groups were fed the above-mentioned diets and were treated with the CsA solvent. As a control, one group was fed only the low-fat diet. The CsA-mediated decreases in brain cholesterol and PC contents, under a low-fat diet, were eliminated by the TG and PC diets. These high-fat diets induced a global increase in hippocampal transcriptional activity, as revealed by elevated polyadenylated RNA levels. The apoE and apoJ mRNA levels in the cortex and hippocampus of rats receiving the solvent were not statistically different between the TG- and PC-enriched diets but showed important variations compared with the low-fat diet solvent-treated group. A differential effect between the two high-fat diets was observed in the hippocampus, resulting in a significant increase of the apoE to apoJ ratio with the PC diet. The balance between apoE and apoJ is presumed to be important in encephalopathic mechanisms, by its involvement through low levels of brain cholesterol and PC, that might be associated with mental disorders. Our results therefore suggest that diet enrichment with polyunsaturated fat might be beneficial during CsA therapy. However, if the high levels in PC used here are more beneficial on CsA peripheral side effects than similar enrichment in TG, this does not seem to be the case in the brain. Thus, lower levels in PC should be tested.

Phosphatidylcholine-enriched diet prevents gallstone formation in mice susceptible to cholelithiasis

Cholesterol gallstones affect approximately 10-15% of the adult population in North America. Phosphatidylcholine (PC) is considered to be the main cholesterol solubilizer in bile. This study examined the effect of a PC-enriched diet on gallstone incidence in mice susceptible to cholelithiasis. The result obtained showed that the feeding of a lithogenic (LG) diet for 4 weeks or 8 weeks resulted in cholesterol gallstone incidences of 47% and 89%, respectively. These gallstone incidences were either reduced or prevented when the LG diet was enriched with 2% or 6% PC, respectively. The cholesterol saturation index (CSI) was reduced only in mice fed with LG + 6% PC diet as compared with mice fed the LG diet alone. However, in all groups, the CSI was significantly higher than in mice fed Purina chow diet. The biliary anionic polypeptide fraction (APF) was significantly increased in mice fed the LG + 2% PC diet and was reduced in those fed with LG + 6% PC diet. In conclusion, prevention or delay of gallstone formation was not due to a consistent effect on biliary lipid composition, suggesting a direct effect of PC on cholesterol solubilization and/or the effect of an additional nonlipid biliary component such as APF.

Biliary anionic peptide fraction and apoA-I regulate intestinal cholesterol uptake

Evidence is now in favor of protein-facilitated mechanisms for the intestinal cholesterol absorption. Here we report that the unesterified cholesterol uptake by rat jejunal brush border membrane vesicles (BBMVs) is efficient, saturable, and protein-mediated. The human apolipoproteins biliary anionic peptide factor (APF) and A-I (apoA-I) up-regulate micellar cholesterol uptake in a dose-dependent manner, but for all tested concentrations (0.1-20 microM), the lipid-free APF was more efficient than apoA-I. This uptake stimulation was suppressed after addition of Pabs directed to the external lipid-binding domain of the CLA-1/SR-BI and reduced by Pabs directed to the external loop of CD36. Thus, CLA-1/SR-BI and to a lesser extent CD36 are involved in the regulation of intestinal cholesterol uptake. APF, the main protein bound to biliary lipids, is likely one of their physiological effectors. As APF is an unesterified cholesterol carrier, it could facilitate the intestinal absorption of biliary cholesterol.

Intralipid 10%: physicochemical characterization

OBJECTIVES

Parenteral fat emulsions contain two populations of particles: artificial chylomicrons rich in triacylglycerols (TAG), and liposomes (bilayer of phospholipids [PL] enveloping an aqueous phase). Centrifugation permits isolating the liposomes in the infranatant called mesophase. The aim of the present work was to better characterize this mesophase chemically and to view the particles it contains by electron microscopy.

METHODS

Electron microscopy (Philips 410) was performed after cryofracture on native 10% Intralipid, mesophase (centrifugation for 1 h at 27 000 g), and a liposome-enriched fraction (ring of density 1.010-1.030 g/l obtained after centrifuging mesophase in a KBr density gradient at 100 000 g for 24 h). The TAG and protein content of the mesophase was analyzed and the proteins partially characterized by immunodetection (Western-blot).

RESULTS

This electron microscope study of 10% Intralipid gives evidence for the coexistence of artificial chylomicrons (mean diameter, 260 nm) and liposomes (43 nm), the latter being smaller than expected and containing 8% w/w TAG after purification. The solubilization of TAG in PL bilayers (reported to be < or = 3.1% w/w) might have been increased in parenteral emulsions by the manufacturing process or/and the high TAG/PL ratio. Minute amounts of proteins have also been detected and partially characterized using a specific antibody raised against the human 7 kDa Anionic Polypeptide Factor (APF), known to strongly interact with PL in bile.

CONCLUSIONS

This work has shown that the size (mean diameter, 43 nm) of the liposomes present in 10% Intralipid is smaller than that usually assumed. Traces of hydrophobic proteins in the emulsion may account for certain allergic reactions sometimes observed in infused patients.

Biliary anionic peptide fraction/calcium binding protein inhibits apolipoprotein A-I-mediated cholesterol efflux from cultured cells

The ABC transporter ABCA1 has been implicated to control cholesterol efflux in a variety of cell types including macrophages, fibroblasts, and intestinal epithelial cells. In this study we have investigated whether the 6-kD protein anionic peptide fraction/calcium binding protein (APF/CBP) which has homology to apolipoprotein AI may regulate efflux mediated by lipoproteins. APF/CBP was purified from T-tube bile by ultracentrifugation and preparative reversed phase HPLC. Cholesterol efflux to a variety of acceptors was determined using cultured fibroblasts from controls and patients with Tangiers disease. APF/CBP (0.1 to 2.4 microg/ml) inhibited ApoA-1 (2 microg/ml) mediated cholesterol efflux from normal fibroblasts in a dose dependent manner but had no effect on aspecific efflux to methyl-beta-cyclodextrin or phosphatidylcholine liposomes. In ABCA1 deficient fibroblasts no effect of APF/CBP on efflux was seen. We conclude that APF/CBP specifically interferes with ApoA-I mediated cholesterol trafficking. We hypothesize that competitive binding to ABCA1 may explain the decreased ApoA-I mediated efflux from fibroblasts.

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.

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.

Overexpression of the HDL receptor SR-BI alters plasma HDL and bile cholesterol levels

The risk of atherosclerosis, a leading cause of cardiovascular disease and death, is inversely related to plasma levels of high-density lipoprotein (HDL) cholesterol, although the mechanism of this protective effect is unclear. The class B scavenger receptor, SR-BI, is the first HDL receptor to be well defined at a molecular level and is a mediator of selective cholesterol uptake in vitro. It is expressed most abundantly in steroidogenic tissues, where it is coordinately regulated with steroidogenesis by adrenocorticotropic hormone (ACTH), human chorionic gonadotropin (hCG) and oestrogen, and in the liver, where its expression in rats is suppressed by oestrogen. Here we show that adenovirus-mediated, hepatic overexpression of SR-BI in mice on both sinusoidal and canalicular surfaces of hepatocytes results in the virtual disappearance of plasma HDL and a substantial increase in biliary cholesterol. SR-BI may directly mediate these effects by increasing hepatic HDL cholesterol uptake or by increasing cholesterol secretion into bile, or both. These results indicate that SR-BI may be important in hepatic HDL metabolism, in determining plasma HDL concentrations, and in controlling cholesterol concentrations in bile, and thus may influence the development and progression of atherosclerosis and gallstone disease.

Biliary secretion of anionic polypeptide fraction is not coupled to that of phospholipids and cholesterol in rats

Anionic polypeptide fraction (APF) is a phospholipid- and calcium-binding apoprotein present in animal and human bile, predominantly associated with cholesterol-phospholipid vesicles. In bile, the protein may play a physiological role in preventing precipitation of calcium salts. APF has also been suggested to be of regulatory importance in the process of biliary lipid secretion. The aim of the present study was to investigate whether the secretion rates of APF and that of biliary lipids are coupled, which would support a physiological role of APF in biliary lipid secretion. Biliary secretion rates of bile acids, phospholipids, and cholesterol were experimentally modulated in three different rat models. Secretion rates of APF were compared with that of bile acids, lipids, and with that of two other biliary proteins, the lysosomal protein beta-glucuronidase and apolipoprotein A-I (apo A-I). Model 1: diurnal variation in bile formation during chronic bile diversion; model 2: specific inhibition of biliary phospholipid and cholesterol, but not of bile acid secretion by infusion of the organic anion, sulfated lithocholyltaurine; model 3: acute interruption of the enterohepatic circulation in unanesthetized rats. The diurnal variation in bile formation involved a parallel increase of the biliary secretion rates of bile acids (+56 +/- 7%, mean +/- SD), phospholipids (+53 +/- 29%), cholesterol (+73 +/- 54%), and APF (+72 +/- 86%) during the night phase of the cycle. Infusion of sulfated lithocholyltaurine inhibited biliary phospholipid and cholesterol secretion (-78 +/- 15%, and -54 +/- 25%, respectively), but did not affect biliary bile acid or APF secretion rate (-19 +/- 14%, and +12 +/- 107%, respectively). Within 4 hours after interruption of the enterohepatic circulation, bile secretion rates for bile acids (-92 +/- 3%), phospholipids (-74 +/- 13%), cholesterol (-64 +/- 8%), and APF (-58 +/- 24%) rapidly declined to a new steady-state level. Correlation analysis using the data from the three experimental models indicated that the biliary secretion rate of APF was independent from that of phospholipids, cholesterol, beta-glucuronidase, and, presumably, apolipoprotein A-I, and positively correlated to bile acid secretion rate and bile flow. The data from three experimental models indicate that the biliary secretion rates of APF and of phospholipids/cholesterol are not coupled and, therefore, do not support a direct physiological role of APF secretion in biliary lipid secretion. APF secretion into bile may, at least partially, be controlled by biliary bile acid secretion.

Distinct immuno-localization of mucin and other biliary proteins in human cholesterol gallstones

BACKGROUND/AIMS

Cholesterol gallstones consist of cholesterol crystals and smaller amounts of pigments and calcium salts, arrayed on a mucin plus protein matrix. The localization of the various biliary proteins in the stones has not been characterized. We aimed to localize several biliary proteins in gallstones in order to determine their possible role in stone formation and growth.

METHODS

The distribution of several matrix proteins and their relationships to the minerals were determined using immunostaining and EDAX microanalysis on hemisected cholesterol gallstones.

RESULTS

Pigment areas were rich in calcium and contained Cu, P and S. These elements were absent in cholesterol regions. Mucin was identified in a three-dimensional network intercalated between cholesterol crystals and as septa between deposits of pigments and cholesterol; APF/CBP and ApN coated only the pigment deposits. No specific topographical localization was found for albumin or IgA.

CONCLUSIONS

This suggests a role for mucin, APF/ CBP and ApN in the formation of cholesterol gallstones. We propose that cholesterol crystals bind directly to mucin, whereas calcium salts and pigments deposit on APF/CBP and ApN bind to the mucin.

Differences in hypolipidaemic effects of two statins on Hep G2 cells or human hepatocytes in primary culture

1. The objective of this study was to compare in cultured human hepatocytes or Hep G2 cells, changes in the fate of unesterified low density lipoprotein (LDL)-cholesterol induced by crilvastatin, a new cholesterol lowering drug and a reference statin, simvastatin. 2. The experiments were carried out for 20 h, each well contained 4.2 x 10(5)/cm2 Hep G2 cells or 0.5 x 10(5)/Cm2 human hepatocytes, 130 microM ursodeoxycholate, 0.68 microCi or 1.59 microCi unesterified human [14C]-LDL-cholesterol, crilvastatin or simvastatin at 0 or 50 microM (both cell types) or 300 microM (Hep-G2 cells). Incubation with the two drugs resulted in increased amounts of unesterified [14C]-LDL-cholesterol taken by the two cell types, compared to control. 3. Crilvastatin 50 microM led to significantly higher quantities of [14C]-glyco-tauro-conjugated bile salts, compared to simvastatin. Statins reduced the apo B100 level secreted by the two cell types (simvastatin) or human hepatocytes (crilvastatin). Crilvastatin enhanced both the level of apo A1 secreted by the Hep G2 cells and the level of APF, a high density lipoprotein (HDL) and biliary apoprotein. 4. Crilvastatin not only acts by stimulating LDL-cholesterol uptake by hepatocytes, but also by enhancing the catabolism of LDL-cholesterol in bile salts and probably by stimulating HDL and/or bile component secretion. Such a mechanism was not previously described for HMG CoA reductase inhibitors. Our results on APF show that this apoprotein could be considered also as an indicator of changes in bile and/or HDL compartments. 5. The human hepatocyte model appeared to be a suitable and relevant model in the pharmacological-metabolic experiments carried out in this study. It led to more consistent data than those obtained with Hep G2 cells.

Phospholipid-rich particles in commercial parenteral fat emulsions. An overview

In parenteral nutrition, the infusion of a fat EMU supplies both concentrated energy and covers the essential fatty acid requirements, the basic objective being to mimic as well as possible the input of chylomicrons into the blood. This objective is well met by the TAGRP of the EMU, which behave as true chylomicrons. However, commercial EMU also contain an excess of emulsifier in the form of PLRP. The number of these PLRP depends directly on the PL/TAG ratio of the EMU. They differ from the TAGRP by their composition (PL vs TAG and PL), their structure (PL in bilayer versus monolayer), and their granulometry (mean diameter 70-100 nm for PL vs 200-500 nm). The metabolic fate of the PLRP is similar in several ways to that of the TAGRP: exchanges of PL with the PL of the different cellular membranes and of the lipoproteins; captation of free CH from these same structures; and enrichment in apolipoproteins. However, because the TAGRP are the preferred substrates of the lipolytic enzymes, their clearance is much more rapid (half-life < 1 h) than that of the PLRP. As the infusion is continued, the PLRP end up accumulating and being transformed into LP-X (free CH/PL = 1; half-life of several days). As soon as the EMU is infused, the PLRP enter into competition with the TAGRP, in the lipolysis process as well as for sites of binding and for catabolism. The sites for catabolism of the two types of PAR are not the same: adipose tissues and muscles utilize the fatty acids and monoacylglycerols released by the lipolysis of the TAGRP; hepatocytes take up their remnants; the RES and the hepatocytes participate in the catabolism of the PLRP and the LP-X. Thus, prolonged infusion of EMU rich in PLRP leads to a hypercholesterolemia, or at least a dyslipoproteinemia, due to elevated LP-X, associated with a depletion of cells in CH, stimulating thus tissue cholesterogenesis. However, parenteral nutrition has evolved towards the utilization of EMU with a low PL/TAG ratio (availability of 30% formula) and less rapid delivery. For these reasons, the hypercholesterolemias that used to be observed with the 10% EMU have become much less spectacular or have even disappeared. It is interesting to note that patients on prolonged TPN, in particular those with a short small intestine, have weak cholesterolemia, reflecting a lowering of HDL and LDL not masked by elevated LP-X. At present, it seems difficult to produce sufficiently stable parenteral EMU devoid of PLRP. Notwithstanding, all the observations made since the introduction of the EMU in TPN are in favour of the use of PLRP-poor EMU. It is clear that the 10% formulas, and generally those with a PL/TAG ratio of 12/100, are ill-advised, especially in patients with a retarded clearance of circulating lipids.

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.

The distribution of the biliary-anionic polypeptide fraction between cholesterol carriers in bile and its effect on nucleation

The small (7 kD) biliary phospholipid and calcium binding polypeptide (anionic polypeptide fraction/calcium binding protein) has been found in higher concentrations in the bile of patients with pigment stones than in controls. In different model systems it was variously found to promote or retard cholesteral crystalization. In the present study we investigated its distribution between cholesterol carriers in bile and its effect on cholesterol crystalization in native and model biles. On gel chromatography anionic polypeptide fraction/calcium binding protein was found predominantly in three areas: in the vesicular fraction, in the non-vesicular lipid fraction and in another fraction unassociated with biliary lipids. It was much more concentrated in the vesicular than in the non-vesicular fraction, the mean anionic polypeptide fraction/phospholipid molar ratio being 219 +/- 181 vs. 30.4 +/- 16, respectively. Anionic polypeptide fraction/calcium binding protein was added at three dose levels, 0.14, 0.28, 0.42 mg/ml (representing approximately 18%-55% of the physiologic biliary concentration), to 19 human and five model biles. This did not produce any significant changes in the nucleation time. The addition of anionic polypeptide fraction/calcium binding protein at a dose level of 0.42 mg/ml to 13 different human biles did not induce changes in the distribution of cholesterol among its carriers. The present experiments do not support a role for anionic polypeptide fraction/calcium binding protein in the process of cholesterol nucleation in bile. Qualitative changes in the protein molecule, as demonstrated in other human secretions, cannot be excluded.

Modulating effects of bile salt hydrophobicity on bile secretion of the major protein of the bile lipoprotein complex

Bile lipids are secreted in association with a newly identified major apoprotein called anionic polypeptide fraction-calcium binding protein (APF-CBP), which is synthesized in the hepatocytes and has been detected in both bile and plasma and characterized. The secretion of the lipids in bile depends both on the concentration and the hydrophobicity of the bile salts (BS) secreted. The present study was undertaken to determine whether the synthesis and the secretion of APF-CBP are similarly regulated by BS, using two methods. The synthesis and secretion of labelled, newly synthesized APF-CBP by isolated rat hepatocytes were monitored by solid-phase immunoassay. For this purpose, hepatocytes were incubated with either glycodeoxycholate (GDC) or taurocholate (TC). The synthesis and secretion of labelled, newly synthesized APF-CBP by perfused rat liver were measured by immunological enzyme-linked assay (ELISA) upon perfusing the liver with either GDC or TC. We found that (i) the synthesis and the secretion of APF-CBP were increased during either TC or GDC perfusion, but the increase was more pronounced with TC; (ii) in GDC perfusion the APF-CBP levels measured were more closely related to the levels of bile salts and not to phospholipid levels, (iii) when the two bile salts were perfused in reverse order, i.e., first GDC and then TC, the secretion of APF-CBP in bile decreased when GDC was perfused, but increased when TC was perfused. Similar results were obtained in experiments with isolated hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Histological evaluation of the intrahepatic biliary tree in intrahepatic cholesterol stones, including immunohistochemical staining against apolipoprotein A-1

Apolipoprotein A-1 is known to be one of inhibiting factors of cholesterol nucleation in bile, and decreased activity of apolipoprotein A-1 is considered to predispose cholesterol-supersaturated bile to formation of cholesterol crystals. To study the pathogenesis of the intrahepatic formation of cholesterol stones, we examined surgically resected liver specimens from six patients with intrahepatic cholesterol stones and compared the characteristic histopathological features with those of intrahepatic calcium bilirubinate stones, using morphological examination and immunohistochemical staining against apolipoprotein A-1. Morphologically, in all six patients with cholesterol stones the severity of chronic proliferative cholangitis with proliferation of the mucus-producing glandular elements in the walls of the large bile duct or periductal tissues was less extensive than that seen with calcium bilirubinate stones, and cholesterol crystals had formed in the septal and interlobular bile ducts. Immunohistochemically, unlike the normal liver and calcium bilirubinate stone-containing lobes, the hepatocytes and the epithelial lining of the bile ducts and peribiliary glands of the cholesterol stone-containing lobes did not react completely (some of the epithelial cells reacted only faintly) with apolipoprotein A-1 antibody. These findings suggest that an abundance of mucous substance and bacterial infection of the biliary tree may not be necessary for the formation of cholesterol stones, compared with findings in cases of calcium bilirubinate stones. We suggest that cholesterol crystals may be produced in the septal and interlobular bile ducts in the microenvironment of cholesterol-supersaturated bile and decreased activity of apolipoprotein A-1.