Protective role of biliary cholesterol and phospholipid lamellae against bile acid-induced cell damage

Serum bile acids associate with liver volume in polycystic liver disease and decrease upon treatment with lanreotide

BACKGROUND

Polycystic liver disease (PLD) is a common extrarenal manifestation of autosomal dominant polycystic kidney disease (ADPKD). Bile acids may play a role in PLD pathogenesis. We performed a post-hoc exploratory analysis of bile acids in ADPKD patients, who had participated in a trial on the effect of a somatostatin analogue. Our hypothesis was that serum bile acid levels increase in PLD, and that lanreotide, which reduces liver growth, may also reduce bile acid levels. Furthermore, in PLD, urinary excretion of bile acids might contribute to renal disease.

METHODS

With liquid chromatography-mass spectrometry, 11 bile acids in serum and 6 in urine were quantified in 105 PLD ADPKD patients and 52 age-, sex-, mutation- and eGFR-matched non-PLD ADPKD patients. Sampling was done at baseline and after 120 weeks of either lanreotide or standard care.

RESULTS

Baseline serum levels of taurine- and glycine-conjugated bile acids were higher in patients with larger livers. In PLD patients, multiple bile acids decreased upon treatment with lanreotide but remained stable in untreated subjects. Changes over time did not correlate with changes in liver volume. Urine bile acid levels did not change and did not correlate with renal disease progression.

CONCLUSION

In ADPKD patients with PLD, baseline serum bile acids were associated with liver volume. Lanreotide reduced bile acid levels and has previously been shown to reduce liver volume. However, in this study, the decrease in bile acids was not associated with the change in liver volume.

Taurohyodeoxycholic acid alleviates trinitrobenzene sulfonic acid induced ulcerative colitis via regulating Th1/Th2 and Th17/Treg cells balance

AIMS

Taurohyodeoxycholic acid (THDCA), a natural 6α-hydroxylated bile acid, exhibits intestinal anti-inflammatory effects. This study aimed to explore the efficacy of THDCA on ulcerative colitis and to reveal its mechanisms of action.

MAIN METHODS

Colitis was induced by intrarectal administration of trinitrobenzene sulfonic acid (TNBS) to mice. Mice in the treatment group were gavage THDCA (20, 40, and 80 mg/kg/day) or sulfasalazine (500 mg/kg/day) or azathioprine (10 mg/kg/day). The pathologic markers of colitis were comprehensively assessed. The levels of Th1-/Th2-/Th17-/Treg-related inflammatory cytokines and transcription factors were detected by ELISA, RT-PCR, and Western blotting. The balance of Th1/Th2 and Th17/Treg cells was analyzed by Flow cytometry.

KEY FINDINGS

THDCA significantly alleviated colitis by improving the body weight, colon length, spleen weight, histological characteristics, and MPO activity of colitis mice. THDCA reduced the secretion of Th1-/Th17-related cytokines (IFN-γ, IL-12p70, IL-6, IL-17A, IL-21, IL-22, and TNF-α) and the expressions of transcription factors (T-bet, STAT4, RORγt, and STAT3), but increase the production of Th2-/Treg-related cytokines (IL-4, IL-10, and TGF-β1) and the expressions of transcription factors (GATA3, STAT6, Foxp3, and Smad3) in the colon. Meanwhile, THDCA inhibited the expressions of IFN-γ, IL-17A, T-bet, and RORγt, but improved the expression of IL-4, IL-10, GATA3, and Foxp3 in the spleen. Furthermore, THDCA restored the proportion of Th1, Th2, Th17, and Treg cells, and balanced the Th1/Th2 and Th17/Treg immune response of colitis mice.

SIGNIFICANCE

THDCA can alleviate TNBS-induced colitis via regulating Th1/Th2 and Th17/Treg balance, which may represent a promising treatment for patients with colitis.

Growth performance, bile acid profile, fecal microbiome and serum metabolomics of growing-finishing pigs fed diets with bile acids supplementation

The present experiment was conducted to determine the effect of bile acids (BAs) supplementation on growth performance, BAs profile, fecal microbiome, and serum metabolomics in growing-finishing pigs. A total of 60 pigs [Duroc × (Landrace × Yorkshire)] with an average body weight of 27.0 ± 1.5 kg were selected and allotted into one of 2 groups (castrated male to female ratio = 1:1), with 10 replicates per treatment and 3 pigs per replicate. The 2 treatments were the control group (control) and a porcine bile extract-supplemented group dosed at 0.5 g/kg feed (BA). After a 16-wk treatment, growth performance, BAs profiles in serum and feces, and fecal microbial composition were determined. An untargeted metabolomics approach using gas chromatography with a time-of-flight mass spectrometer was conducted to identify the metabolic pathways and associated metabolites in the serum of pigs. We found that BAs supplementation had no effect on the growth performance of the growing-finishing pig. However, it tended to increase the gain-to-feed ratio for the whole period (P = 0.07). BAs supplementation resulted in elevated serum concentrations of secondary bile acids, including hyodeoxycholic acid (HDCA), glycoursodeoxycholic acid, and tauro-hyodeoxycholic acid, as well as fecal concentration of HDCA (P < 0.05). Fecal microbiota analysis revealed no differences in alpha and beta diversity indices or the relative abundance of operational taxonomic units (OTUs) at both phylum and genus levels between groups. Metabolic pathway analysis revealed that the differential metabolites between control and BA groups are mainly involved in purine metabolism, ether lipid metabolism, glycerophospholipid metabolism, and amino sugar and nucleotide sugar metabolism, as well as primary bile acid biosynthesis. Our findings indicate that BAs supplementation tended to improve the feed efficiency, and significantly altered the BA profile in the serum and feces of growing-finished pigs, regardless of any changes in the gut microbial composition. The altered metabolic pathways could potentially play a vital role in improving the feed efficiency of growing-finished pigs with BAs supplementation.

Liver regeneration observed across the different classes of vertebrates from an evolutionary perspective

The liver is a key organ that performs diverse functions such as metabolic processing of nutrients or disposal of dangerous substances (xenobiotics). Accordingly, it seems to be protected by several mechanisms throughout the life of organisms, one of which is compensatory hyperplasia, also known as liver regeneration. This review is a recapitulation of the scientific reports describing the different ways in which the various classes of vertebrates deal with liver injuries, where since mammals have an improved molecular toolkit, exhibit optimized regeneration of the liver compared to lower vertebrates. The main molecules involved in the compensatory process, such as proinflammatory and inhibitory cytokines, are analyzed across vertebrates with an evolutionary perspective. In addition, the possible significance of this mechanism is discussed in the context of the long life span of vertebrates, especially in the case of mammals.

Stimulation of ABCB4/MDR3 ATPase activity requires an intact phosphatidylcholine lipid

ABCB4/MDR3 is located in the canalicular membrane of hepatocytes and translocates PC-lipids from the cytoplasmic to the extracellular leaflet. ABCB4 is an ATP-dependent transporter that reduces the harsh detergent effect of the bile salts by counteracting self-digestion. To do so, ABCB4 provides PC lipids for extraction into bile. PC lipids account for 40% of the entire pool of lipids in the canalicular membrane with an unknown distribution over both leaflets. Extracted PC lipids end up in so-called mixed micelles. Mixed micelles are composed of phospholipids, bile salts, and cholesterol. Ninety to ninety-five percent of the phospholipids are members of the PC family, but only a subset of mainly 16.0-18:1 PC and 16:0-18:2 PC variants are present. To elucidate whether ABCB4 is the key discriminator in this enrichment of specific PC lipids, we used in vitro studies to identify crucial determinants in substrate selection. We demonstrate that PC-lipid moieties alone are insufficient for stimulating ABCB4 ATPase activity, and that at least two acyl chains and the backbone itself are required for a productive interaction. The nature of the fatty acids, like length or saturation has a quantitative impact on the ATPase activity. Our data demonstrate a two-step enrichment and protective function of ABCB4 to mitigate the harsh detergent effect of the bile salts, because ABCB4 can translocate more than just the PC-lipid variants found in bile.

ABCB4/MDR3 in health and disease – at the crossroads of biochemistry and medicine

Several ABC transporters of the human liver are responsible for the secretion of bile salts, lipids and cholesterol. Their interplay protects the biliary tree from the harsh detergent activity of bile salts. Among these transporters, ABCB4 is essential for the translocation of phosphatidylcholine (PC) lipids from the inner to the outer leaflet of the canalicular membrane of hepatocytes. ABCB4 deficiency can result in altered PC to bile salt ratios, which led to intrahepatic cholestasis of pregnancy, low phospholipid associated cholelithiasis, drug induced liver injury or even progressive familial intrahepatic cholestasis type 3. Although PC lipids only account for 30–40% of the lipids in the canalicular membrane, 95% of all phospholipids in bile are PC lipids. We discuss this discrepancy in the light of PC synthesis and bile salts favoring certain lipids. Nevertheless, the in vivo extraction of PC lipids from the outer leaflet of the canalicular membrane by bile salts should be considered as a separate step in bile formation. Therefore, methods to characterize disease causing ABCB4 mutations should be considered carefully, but such an analysis represents a crucial point in understanding the currently unknown transport mechanism of this ABC transporter.

Alteration in bile acids profile in Large White pigs during chronic heat exposure

Bile acids (BAs) are critical for cholesterol homeostasis and new roles in metabolism and endocrinology have been demonstrated recently. It remains unknown whether BA metabolism can be affected by heat stress (HS). The objective of this study was to describe the shifts in serum, hepatic and intestinal BA profiles induced by chronic HS. Twenty-seven Large White pigs weighing 40.8 ± 2.7 kg were assigned to one of the three treatments: a control group (CON, 23 °C), a HS group (33 °C), or a pair-fed group (PF, 23 °C and fed the same amount as HS group) for 21 d. The concentrations of taurine-conjugated BAs (TUDCA and THDCA in serum and TCDCA, TUDCA, THDCA and THCA in liver) were decreased in HS and PF pigs. However, in HS pigs, a reduction in taurine-conjugated BAs (TCBA) correlated with decreased liver genes expression of BA synthesis, conjugation and uptake transport. BA regulated-genes (FXR, TGR5 and FGFR4) in HS pigs and TGR5, FGFR4 and KLβ in PF pigs were down-regulated in liver. In ileum, total BAs and glycoursodeoxycholic acid concentrations were higher in HS pigs than other groups and PF group, respectively (P < 0.05). TCBA (P = 0.01) and tauroursodeoxycholic acid (P < 0.01) were decreased in PF group. BA transporters (OSTα and MRP3) were up-regulated in HS pigs compared with CON and PF pigs, respectively (P < 0.01). In cecum, ursodeoxycholic acid was higher in HS (P = 0.02) group than CON group. The expression of apical sodium-coupled bile acid transporter (P = 0.04) was lower in HS pigs than CON pigs, while OSTβ (P < 0.01) was greater in HS group than PF group. These results suggest that chronic HS suppressed liver activity of synthesis and uptake of TCBA, at least in part, which was independent of reduced feed intake.

Biliary bile acids in hepatobiliary injury - What is the link?

The main trigger for liver injury in acquired cholestatic liver disease remains unclear. However, the accumulation of bile acids (BAs) undoubtedly plays a role. Recent progress in deciphering the pathomechanisms of inborn cholestatic liver diseases, decoding mechanisms of BA-induced cell death, and generating modern BA-derived drugs has improved the understanding of the regulation of BA synthesis and transport. Now is the appropriate time to reassess current knowledge about the specific role of BAs in hepatobiliary injury.

Cholesterol attenuates cytoprotective effects of phosphatidylcholine against bile salts

Bile salts have potent detergent properties and damaging effects on cell membranes, leading to liver injury. However, the molecular mechanisms for the protection of hepatocytes against bile salts are not fully understood. In this study, we demonstrated that the cytotoxicity of nine human major bile salts to HepG2 cells and primary human hepatocytes was prevented by phosphatidylcholine (PC). In contrast, cholesterol had no direct cytotoxic effects but suppressed the cytoprotective effects of PC. PC reduced the cell-association of bile salt, which was reversed by cholesterol. Light scattering measurements and gel filtration chromatography revealed that cholesterol within bile salt/PC dispersions decreased mixed micelles but increased vesicles, bile salt simple micelles and monomers. These results suggest that cholesterol attenuates the cytoprotective effects of PC against bile salts by facilitating the formation of bile salt simple micelles and monomers. Therefore, biliary PC and cholesterol may play different roles in the pathogenesis of bile salt-induced liver injury.

The ascending pathophysiology of cholestatic liver disease

In this review we develop the argument that cholestatic liver diseases, particularly primary biliary cholangitis and primary sclerosing cholangitis (PSC), evolve over time with anatomically an ascending course of the disease process. The first and early lesions are in "downstream" bile ducts. This eventually leads to cholestasis, and this causes bile salt (BS)-mediated toxic injury of the "upstream" liver parenchyma. BS are toxic in high concentration. These concentrations are present in the canalicular network, bile ducts, and gallbladder. Leakage of bile from this network and ducts could be an important driver of toxicity. The liver has a great capacity to adapt to cholestasis, and this may contribute to a variable symptom-poor interval that is often observed. Current trials with drugs that target BS toxicity are effective in only about 50%-60% of primary biliary cholangitis patients, with no effective therapy in PSC. This motivated us to develop and propose a new view on the pathophysiology of primary biliary cholangitis and PSC in the hope that these new drugs can be used more effectively. These views may lead to better stratification of these diseases and to recommendations on a more "tailored" use of the new therapeutic agents that are currently tested in clinical trials. Apical sodium-dependent BS transporter inhibitors that reduce intestinal BS absorption lower the BS load and are best used in cholestatic patients. The effectiveness of BS synthesis-suppressing drugs, such as farnesoid X receptor agonists, is greatest when optimal adaptation is not yet established. By the time cytochrome P450 7A1 expression is reduced these drugs may be less effective. Anti-inflammatory agents are probably most effective in early disease, while drugs that antagonize BS toxicity, such as ursodeoxycholic acid and nor-ursodeoxycholic acid, may be effective at all disease stages. Endoscopic stenting in PSC should be reserved for situations of intercurrent cholestasis and cholangitis, not for cholestasis in end-stage disease. These are arguments to consider a step-wise pathophysiology for these diseases, with therapy adjusted to disease stage. An obstacle in such an approach is that disease stage-defining biomarkers are still lacking. This review is meant to serve as a call to prioritize the development of biomarkers that help to obtain a better stratification of these diseases. (Hepatology 2017;65:722-738).

Comparison of Laboratory Data of Acute Cholangitis Patients Treated with or without Immunosuppressive Drugs

Objective. Symptoms and laboratory data between acute cholangitis (AC) patients treated with and AC patients treated without immunosuppressive drugs (corticosteroids or methotrexate) were compared to identify factors that can be meaningful to the diagnosis of AC. Methods. The Wilcoxon signed-rank test was used for comparison of baseline variables between the patients with AC treated with immunosuppressive drugs and those without it. The chi-squared test was used in the analysis of the symptoms. Results. In total, 69 patients with AC were enrolled. Fifteen patients were treated with immunosuppressants due to rheumatoid arthritis or other collagen diseases. Jaundice was less frequent in the patients treated with immunosuppressive drugs (P = 0.0351). T-Bil level was marginally lower in the patients treated with immunosuppressants (P = 0.086). AST and ALT levels were lower in the patients treated with immunosuppressants (P = 0.0417 and 0.022, respectively). Conclusions. The frequency of jaundice and AST and ALT levels were lower in the patients treated with immunosuppressive drugs. It is recommended that care be taken to evaluate jaundice, AST level, and ALT level in the diagnosis of AC.

A "forward genomics" approach links genotype to phenotype using independent phenotypic losses among related species

Genotype-phenotype mapping is hampered by countless genomic changes between species. We introduce a computational "forward genomics" strategy that-given only an independently lost phenotype and whole genomes-matches genomic and phenotypic loss patterns to associate specific genomic regions with this phenotype. We conducted genome-wide screens for two metabolic phenotypes. First, our approach correctly matches the inactivated Gulo gene exactly with the species that lost the ability to synthesize vitamin C. Second, we attribute naturally low biliary phospholipid levels in guinea pigs and horses to the inactivated phospholipid transporter Abcb4. Human ABCB4 mutations also result in low phospholipid levels but lead to severe liver disease, suggesting compensatory mechanisms in guinea pig and horse. Our simulation studies, counts of independent changes in existing phenotype surveys, and the forthcoming availability of many new genomes all suggest that forward genomics can be applied to many phenotypes, including those relevant for human evolution and disease.

Ursodeoxycholic acid in cholestasis: linking action mechanisms to therapeutic applications

UDCA (ursodeoxycholic acid) is the therapeutic agent most widely used for the treatment of cholestatic hepatopathies. Its use has expanded to other kinds of hepatic diseases, and even to extrahepatic ones. Such versatility is the result of its multiple mechanisms of action. UDCA stabilizes plasma membranes against cytolysis by tensioactive bile acids accumulated in cholestasis. UDCA also halts apoptosis by preventing the formation of mitochondrial pores, membrane recruitment of death receptors and endoplasmic-reticulum stress. In addition, UDCA induces changes in the expression of metabolizing enzymes and transporters that reduce bile acid cytotoxicity and improve renal excretion. Its capability to positively modulate ductular bile flow helps to preserve the integrity of bile ducts. UDCA also prevents the endocytic internalization of canalicular transporters, a common feature in cholestasis. Finally, UDCA has immunomodulatory properties that limit the exacerbated immunological response occurring in autoimmune cholestatic diseases by counteracting the overexpression of MHC antigens and perhaps by limiting the production of cytokines by immunocompetent cells. Owing to this multi-functionality, it is difficult to envisage a substitute for UDCA that combines as many hepatoprotective effects with such efficacy. We predict a long-lasting use of UDCA as the therapeutic agent of choice in cholestasis.

Importance of biliary excretion of indomethacin in gastrointestinal and hepatic injury

BACKGROUND AND AIMS

A mechanism for protection of gastrointestinal (GI) and hepatic cells from damaging detergent actions of bile acids appears to involve the bile component, phosphatidylcholine (PC). Non-steroidal anti-inflammatory drugs (NSAIDs) induce intestinal injury in direct proportion to their ability to be excreted into bile, and are known to chemically associate with PC. We investigated the role of bile acids and PC in the mechanism of indomethacin-induced epithelial injury.

METHODS

Rats were injected orally or intravenously with radiolabeled indomethacin and their bile was collected over time for determination of NSAID secretion. Bile from rats treated with or without indomethacin was used in studies of red blood cell (RBC) hemolysis as a measure of membrane cytotoxicity. The bile salt, sodium deoxycholate (SDC), and indomethacin were tested alone and in combination with PC on RBC and on hepatic HepG2 cells.

RESULTS

Intravenously or orally given indomethacin was quantitatively excreted (approximately 50%) into bile over a 2-h study period. Bile from a rat treated with indomethacin or bile with exogenous indomethacin was cytotoxic to RBC, and the injury was prevented by the addition of PC. Hepatocytes exposed to SDC showed injury that could be dose-dependently prevented by PC, and reversed by indomethacin.

CONCLUSIONS

Biliary PC plays an important physiological role in protecting GI and hepatic epithelia from the cytotoxic actions of bile salts. The ability of NSAIDs excreted into the bile to associate with mixed bile salt micelles and reduce the protective action of the PC may be a critical component in the drugs' pathogenic mechanism.

Hepatocellular transport in acquired cholestasis: new insights into functional, regulatory and therapeutic aspects

The recent overwhelming advances in molecular and cell biology have added enormously to our understanding of the physiological processes involved in bile formation and, by extension, to our comprehension of the consequences of their alteration in cholestatic hepatopathies. The present review addresses in detail this new information by summarizing a number of recent experimental findings on the structural, functional and regulatory aspects of hepatocellular transporter function in acquired cholestasis. This comprises (i) a short overview of the physiological mechanisms of bile secretion, including the nature of the transporters involved and their role in bile formation; (ii) the changes induced by nuclear receptors and hepatocyte-enriched transcription factors in the constitutive expression of hepatocellular transporters in cholestasis, either explaining the primary biliary failure or resulting from a secondary adaptive response; (iii) the post-transcriptional changes in transporter function and localization in cholestasis, including a description of the subcellular structures putatively engaged in the endocytic internalization of canalicular transporters and the involvement of signalling cascades in this effect; and (iv) a discussion on how this new information has contributed to the understanding of the mechanism by which anticholestatic agents exert their beneficial effects, or the manner in which it has helped the design of new successful therapeutic approaches to cholestatic liver diseases.

Role of phosphatidylcholine saturation in preventing bile salt toxicity to gastrointestinal epithelia and membranes

BACKGROUND AND AIM

The mechanism which protects the biliary and intestinal mucosa from the detergent properties of bile acids is not fully understood. We employed three contrasting in vitro model systems (human red blood cells, polarized intestinal [Caco-2] cells, and synthetic liposomes), to compare the efficacy of saturated and unsaturated phosphatidylcholine (PC) to protect cells and membranes from bile salt injury.

METHODS

Hemolysis of red blood cells, electrical resistance across confluent monolayers of Caco-2 cells, and disruption of synthetic PC liposomes were assessed after incubation with varying concentrations of bile salt (sodium deoxycholate) alone or in the presence of saturated or unsaturated PC.

RESULTS

The hemolytic activity of deoxycholate on red blood cells was observed at > or =2 mM, and could be blocked by equimolar concentration or greater of both saturated or unsaturated PC. In contrast, exposure of Caco-2 cells to deoxycholate at > or =0.8 mM induced a maximal decrease in resistance, which was reversed by > or =0.8 mM unsaturated PC or 5 mM saturated PC. Similarly, synthetic liposomes were permeabilized by 0.8 mM deoxycholate and were protected by a lower concentration of unsaturated PC (2 mM) than saturated (5 mM).

CONCLUSIONS

Cells can show variable resistance to bile salt toxicity. Extracellular PC, especially in the unsaturated state, can directly protect cell and artificial membranes from bile salt injury. These findings support a role for biliary PC in the formation of mixed micelles that have low cytotoxic properties.

ME3738 protects against lithocholic acid-induced hepatotoxicity, which is associated with enhancement of biliary bile acid and cholesterol output

ME3738 (22beta-methoxyolean-12-ene-3beta, 24(4beta)-diol), a derivative of soyasapogenol, attenuates liver disease in several models of chronic liver inflammation. In the present study, we have investigated a protective effect of ME3738 in a typical bile acid-induced cholestatic liver model, lithocholate (LCA) feeding mouse. Co-administration of ME3738 resulted in decreases in plasma alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities and hepatic bile acid level, and increases in biliary outputs of bile acid and cholesterol, as compared with the results in mice treated with LCA alone. LCA sulfation by hydroxysteroid sulfotransferase 2a and hydroxylation have been reported to be involved in protection against LCA-induced hepatotoxicity. ME3738-treatment, however, had no clear influence on the hydroxysteroid sulfotransferase 2a protein level and LCA 6alpha-, 6beta- and 7alpha-hydroxylase activities, but increased biliary cholesterol output. Cholate (CA)-treatment has been shown to induce hepatotoxicity in farnesoid X receptor-null mice, which is scarcely dependent on bile acid sulfation and hydroxylation but associated with decreased biliary bile acid output. Co-administration of ME3738 decreased the ALT and ALP activities and hepatic bile acid level, and increased biliary outputs of bile acid and cholesterol in farnesoid X receptor-null mice, as compared with the results in the mice treated with CA. Moreover, a clear correlation between biliary outputs of cholesterol and bile acid was observed in these two bile acid-induced hepatotoxicity mouse models. These results suggest that ME3738 protects against bile acid-induced hepatotoxicity through increased biliary bile acid output that is not related to bile acid metabolism but associated with cholesterol output.

Anti-inflammatory effects of phosphatidylcholine

We recently showed that mucus from patients with ulcerative colitis, a chronic inflammatory disorder of the colon, is characterized by a low level of phosphatidylcholine (PC) while clinical studies reveal that therapeutic addition of PC using slow release preparations is beneficial. The positive role of PC in this disease is still elusive. Here we tested the hypothesis that exogenous application of PC has anti-inflammatory properties using three model systems. First, human Caco-2 cells were treated with tumor necrosis factor-alpha (TNF-alpha) to induce a pro-inflammatory response via activation of NF-kappaB. Second, latex bead phagosomes were analyzed for their ability to assemble actin in vitro, a process linked to pro-inflammatory signaling and correlating with the growth versus killing of mycobacteria in macrophages. The third system used was the rapid assembly of plasma membrane actin in macrophages in response to sphingosine 1-phosphate. TNF-alpha induced a pro-inflammatory response in Caco-2 cells, including 1) assembly of plasma membrane actin; 2) activation of both MAPKs ERK and p38; 3) transport of NF-kappaB subunits to the nucleus; and 4) subsequent up-regulation of the synthesis of pro-inflammatory gene products. Exogenous addition of most PCs tested significantly inhibited these processes. Other phospholipids like sphingomyelin or phosphatidylethanolamine showed no effects in these assays. PC also inhibited latex bead phagosome actin assembly, the killing of Mycobacterium tuberculosis in macrophages, and the sphingosine 1-phosphate-induced actin assembly in macrophages. TNF-alpha induces the activation of signaling molecules and the reorganization of the actin cytoskeleton in human intestinal cells. Exogenous application of PC blocks pro-inflammatory signaling in Caco-2 cells, in phagosomes in vitro and facilitates intracellular survival of mycobacteria. We provide further evidence that actin assembly by membranes is part of the pro-inflammatory response. Collectively, these results provide a molecular foundation for the clinical studies showing a beneficial effect of PC therapy in ulcerative colitis.

Bile acid interactions with cholangiocytes

Cholangiocytes are exposed to high concentrations of bile acids at their apical membrane. A selective transporter for bile acids, the Apical Sodium Bile Acid Cotransporter (ASBT) (also referred to as Ibat; gene name Slc10a2) is localized on the cholangiocyte apical membrane. On the basolateral membrane, four transport systems have been identified (t-ASBT, multidrug resistance (MDR)3, an unidentified anion exchanger system and organic solute transporter (Ost) heteromeric transporter, Ostα-Ostβ. Together, these transporters unidirectionally move bile acids from ductal bile to the circulation. Bile acids absorbed by cholangiocytes recycle via the peribiliary plexus back to hepatocytes for re-secretion into bile. This recycling of bile acids between hepatocytes and cholangiocytes is referred to as the cholehepatic shunt pathway. Recent studies suggest that the cholehepatic shunt pathway may contribute in overall hepatobiliary transport of bile acids and to the adaptation to chronic cholestasis due to extrahepatic obstruction. ASBT is acutely regulated by an adenosine 3', 5’-monophosphate (cAMP)-dependent translocation to the apical membrane and by phosphorylation-dependent ubiquitination and proteasome degradation. ASBT is chronically regulated by changes in gene expression in response to biliary bile acid concentration and inflammatory cytokines. Another potential function of cholangiocyte ASBT is to allow cholangiocytes to sample biliary bile acids in order to activate intracellular signaling pathways. Bile acids trigger changes in intracellular calcium, protein kinase C (PKC), phosphoinositide 3-kinase (PI3K), mitogen-activated protein (MAP) kinase and extracellular signal-regulated protein kinase (ERK) intracellular signals. Bile acids significantly alter cholangiocyte secretion, proliferation and survival. Different bile acids have differential effects on cholangiocyte intracellular signals, and in some instances trigger opposing effects on cholangiocyte secretion, proliferation and survival. Based upon these concepts and observations, the cholangiocyte has been proposed to be the principle target cell for bile acids in the liver.

Diosgenin-induced biliary cholesterol secretion in mice requires Abcg8

The plant sterol diosgenin has been shown to stimulate biliary cholesterol secretion in mice without affecting the expression of the adenosine triphosphate-binding cassette transporter heterodimer Abcg5/g8. The aim of this study was to investigate the mechanism of diosgenin-induced cholesterol hypersecretion and to identify the genes involved. Surprisingly, despite its lack of effect on Abcg5/g8 expression in wild-type mice, diosgenin did not stimulate biliary cholesterol secretion in mice deficient for Abcg8. Analysis of the kinetics of cholesterol secretion suggested that diosgenin probably activates a step before Abcg5/g8. To identify potential diosgenin targets, gene expression profiling was performed in mice fed a diosgenin-supplemented diet. Diosgenin feeding increased hepatic expression of genes involved in cholesterol synthesis as well as genes encoding for several cytochrome P450s. No significant change in expression of known cholesterol transporters was found. Comparison with published expression-profiling data for Srebp2-overexpressing mice, another mouse model in which biliary cholesterol secretion is elevated, revealed a number of genes with unknown function that were upregulated in both diosgenin-fed mice and mice overexpressing Srebp2. In conclusion, we found that although Abcg8 is essential for most diosgenin-induced biliary cholesterol hypersecretion, diosgenin probably does not interact directly with Abcg5/Abcg8, but rather increases cholesterol delivery to the heterodimer. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Spontaneous cholecysto- and hepatolithiasis in Mdr2-/- mice: a model for low phospholipid-associated cholelithiasis

Previously, we identified needle-like and filamentous, putatively "anhydrous" cholesterol crystallization in vitro at very low phospholipid concentrations in model and native biles. Our aim now was to address whether spontaneous gallstone formation occurs in Mdr2 (Abcb4) knockout mice that are characterized by phospholipid-deficient bile. Biliary phenotypes and cholesterol crystallization sequences in fresh gallbladder biles and non-fixed liver sections were determined by direct and polarizing light microscopy. The physical chemical nature and composition of crystals and stones were determined by sucrose density centrifugation and before mass and infrared spectroscopy. Gallbladder biles of Mdr2(-/-) mice precipitate needle-like cholesterol crystals at 12 weeks of age on chow. After 15 weeks, more than 50% of Mdr2(-/-) mice develop gallbladder stones, with female mice displaying a markedly higher gallstone-susceptibility. Although gallbladder biles of Mdr2(-/-) mice contain only traces (</= 1.1 mM) of phospholipid and cholesterol, they become supersaturated with cholesterol and plot in the left 2-phase zone of the ternary phase diagram, consistent with "anhydrous" cholesterol crystallization. Furthermore, more than 40% of adult female Mdr2(-/-) mice show intra- and extrahepatic bile duct stones. In conclusion, spontaneous gallstone formation is a new consistent feature of the Mdr2(-/-) phenotype. The Mdr2(-/-) mouse is therefore a model for low phospholipid-associated cholelithiasis recently described in humans with a dysfunctional mutation in the orthologous ABCB4 gene. The mouse model supports the concept that this gene is a monogenic risk factor for cholesterol gallstones and a target for novel therapeutic strategies.

ABCB4 gene mutation-associated cholelithiasis in adults

BACKGROUND & AIMS

We recently put forward arguments in favor of ABCB4 gene (adenosine triphosphate-binding cassette, subfamily B, member 4) defects as a risk factor for symptomatic cholelithiasis in adults. In this study, we characterized ABCB4 gene mutations in a series of patients with symptomatic cholelithiasis to determine the genetic basis and the clinical phenotype of ABCB4 gene mutation-associated cholelithiasis.

METHODS

We analyzed the entire ABCB4 gene coding sequences in a first group of 32 patients who had a clinical history compatible with the syndrome previously described, in a second group of 28 patients who presented with a classic gallstone disease that justified a cholecystectomy, and in a third group of 33 patients without a history of cholelithiasis.

RESULTS

We identified both heterozygous and homozygous ABCB4 gene point mutations in 18 of 32 (56%) patients who presented with clinical criteria of the syndrome, whereas no mutation was detected in the 2 other groups of patients (P < 0.001). Three independent clinical features were strongly associated with point mutations: recurrence of symptoms after cholecystectomy (odds ratio, 8.5); intrahepatic hyperechoic foci, intrahepatic sludge, or microlithiasis (odds ratio, 6.1); and age <40 years at the onset of symptoms (odds ratio, 3.0). ABCB4 gene point mutations were detected exclusively in the patients who showed 2 or 3 of these clinical features.

CONCLUSIONS

Our results show that ABCB4 gene mutations represent a major genetic risk factor in a symptomatic and recurring form of cholelithiasis in young adults.

Effects of bile acids on biliary epithelial cells: proliferation, cytotoxicity, and cytokine secretion

Hydrophobic bile acids, which are known to be cytotoxic for hepatocytes, are retained in high amount in the liver during cholestasis. Thus, we have investigated the effects of bile acids with various hydrophobicities on biliary epithelial cells. Biliary epithelial cells were cultured in the presence of tauroursodeoxycholate (TUDC), taurocholate (TC), taurodeoxycholate (TDC), taurochenodeoxycholate (TCDC), or taurolithocholate (TLC). Cell proliferation, viability, apoptosis and secretion of monocyte chemotactic protein-1 (MCP-1) and of interleukin-6 (IL-6) were studied. Cell proliferation was increased by TDC, and markedly decreased by TLC in a dose dependent manner (50-500 microM). Cell viability was significantly decreased by TLC and TCDC at 500 microM. TLC, TDC and TCDC induced apoptosis at high concentrations. The secretion of MCP-1 and IL-6 was markedly stimulated by TC. TUDC had no significant effect on any parameter. These findings demonstrate that hydrophobic bile acids were cytotoxic and induced apoptosis of biliary epithelial cells. Furthermore, TC, a major biliary acid in human bile, stimulated secretion of cytokines involved in the inflammatory and fibrotic processes occurring during cholestatic liver diseases.

Comparative cytotoxic and cytoprotective effects of taurohyodeoxycholic acid (THDCA) and tauroursodeoxycholic acid (TUDCA) in HepG2 cell line

This study was performed to compare the effects of two hydrophilic bile acids, taurohyodeoxycholic acid (THDCA) and tauroursodeoxycholic acid (TUDCA), on HepG2 cells. Cytotoxicity was evaluated at different times of exposure by incubating cells with increasing concentrations (50-800 micromol/l) of either bile acid, while their cytoprotective effect was tested in comparison with deoxycholic acid (DCA) (350 micromol/l and 750 micromol/l)-induced cytotoxicity. Culture media, harvested at the end of each incubation period, were analyzed to evaluate aspartate transaminase (AST), alanine transaminase and gamma-glutamyltranspeptidase release. In addition, the hemolytic effect of THDCA and TUDCA on human red blood cells was also determined. At 24 h of incubation neither THDCA nor TUDCA was cytotoxic at concentrations up to 200 and 400 micromol/l. At 800 micromol/l both THDCA and TUDCA induced a slight increase in AST release. At this concentration and with time of exposure prolonged up to 72 h, THDCA and TUDCA induced a progressive increase of AST release significantly (P<0.05) higher than that of controls being AST values for THDCA (2.97+/-0.88 time control value (tcv) at 48 h and 4.50+/-1.13 tcv at 72 h) significantly greater than those of TUDCA (1.50+/-0.20 tcv at 48 h and 1.80+/-0.43 tcv at 72 h) (P<0.01). In cytoprotection experiments, the addition of 50 micromol/l THDCA decreased only slightly (-5%) AST release induced by 350 micromol/l DCA, while the addition of 50 micromol/l TUDCA was significantly effective (-23%; P<0.05). Higher doses of THDCA or TUDCA did not reduce toxicity induced by 350 micromol/l DCA, but were much less toxic than an equimolar dose of DCA alone. At the concentration used in this experimental model neither THDCA nor TUDCA was hemolytic; however at a very high concentration (6 mmol/l) both bile acids induced 5-8% hemolysis. We conclude that bile acid molecules with a similar degree of hydrophilicity may show different cytotoxic and cytoprotective properties.

Regulation of biliary cholesterol secretion is independent of hepatocyte canalicular membrane lipid composition: a study in the diosgenin-fed rat model

BACKGROUND/AIMS

Phosphatidylcholine (PC) and sphingomyelin (SM) are the major phospholipids on the outer leaflet of the hepatocyte canalicular membrane. Since cholesterol preferentially associates with SM in detergent-resistant microdomains, we hypothesized that canalicular membrane lipid composition could modulate secretion of the sterol into bile.

METHODS

Male Wistar rats were fed for 10 days with a control diet with or without the plant sterol diosgenin (1% w/w) to induce biliary cholesterol hypersecretion. Thereafter, lipid compositions and phospholipid molecular species were determined in fistula bile and highly enriched canalicular membrane fractions.

RESULTS

Despite four-fold higher biliary cholesterol output in diosgenin-fed rats, no differences were observed between canalicular membranes of diosgenin and control groups with respect to cholesterol/phospholipid ratios (0.58 vs 0.62), phospholipid classes and acyl chain compositions of SMs (16:0 > 24:1 > 24:0 > 22:0 > 18:0 > 23:0 > 20:0 > 24:2), or PCs (mainly diacyl 16:0-18:2, 16:0-20:4, 18:0-20:4, and 18:0-18:2). In contrast to canalicular PCs, bile contained more hydrophilic species (mainly diacyl 16:0-18:2 and 16:0-20:4), without differences between both groups. In vitro resistance of purified canalicular membrane fractions against detergents such as Triton X-100 and taurocholate was also similar in both groups.

CONCLUSIONS

Diosgenin-induced biliary cholesterol hypersecretion occurs in the absence of changes of canalicular membrane lipids. Our data therefore do not support a major role of canalicular membrane lipid composition in regulation of biliary cholesterol secretion.

Role of perivenous hepatocytes in taurolithocholate-induced cholestasis in vivo

The magnitude of cholestasis induced by taurolithocholic acid (TLCA) and its relationship with phase I metabolism were analyzed in rats treated with bromobenzene (BZ), a chemical that causes selective necrosis of perivenous (zone 3) hepatocytes. Forty-eight hours after BZ administration (600 mg/Kg bw), a single dose of 20 micromol/Kg bw of TLCA was injected. Bile was collected during 180 min and bile flow and total bile acid excretion rate were determined. Biliary bile acid composition was analyzed by gas-liquid chromatography-mass spectrometry. BZ administration did not affect the development of TLCA-induced cholestasis, but exacerbated the bile acid-induced decrease in bile flow during the period of recovery from cholestasis. Biliary excretion of total bile acids after TLCA injection relative to basal value was not effected by BZ. The analysis of bile acid composition in bile revealed that TLCA was partially converted to hyodeoxycholic and muricholic acids. The cumulative excretion of all exogenous bile acids and their contribution to the composition of the biliary bile acid pool were not substantially affected by zone 3 necrosis, suggesting that synthesis and secretion of hydroxylated derivatives of TLCA were maintained by zone 1 and 2 hepatocytes. The relative content of endogenous bile acids was not affected by BZ during TLCA-induced cholestasis. Thus, it seems unlikely that the exacerbation of the cholestasis in BZ-treated rats is due to different choleretic properties and/or toxicity of the bile acid pool.

Role of biliary phosphatidylcholine in bile acid protection and NSAID injury of the ileal mucosa in rats

BACKGROUND & AIMS

We explored the role of biliary phosphatidylcholine (PC) in protection of the intestinal mucosa against bile salt (BS)-induced intestinal injury and how this property may be blocked by indomethacin (Indo), a nonsteroidal anti-inflammatory drug (NSAID) that is secreted into the bile.

METHODS

We performed in vivo studies in which bile was collected over a 2-hour period after rats were intragastrically administered Indo (25 mg/kg) or an equivalent volume of saline (controls). The bile samples (some of which were supplemented with PC) were then instilled into a loop of distal ileum of anesthetized rats. After a 30-minute exposure period, we measured the hemoglobin concentration of the ileal loop fluid, as an index of bleeding, and mucosal contact angles, as an index of surface hydrophobicity. A similar in vivo experiment was performed in which model bile containing 5 mmol/L each of the BS, sodium deoxycholate, PC, or Indo, alone and in combination, was instilled into ileal loops. In our in vitro test system, human erythrocytes were exposed to the above biliary constituents, and hemolysis was measured spectrophotometrically.

RESULTS

Bile from Indo-pretreated rats decreased the surface hydrophobicity and induced bleeding of ileal loops in comparison with control bile, and both NSAID-induced changes were reversed if PC was added to the bile. Similarly, synthetic BS caused gastrointestinal bleeding, decreased ileal contact angles, and induced erythrocyte hemolysis, all of which were reversed by addition of equimolar PC. This protective role of PC in both the in vivo and in vitro systems was partially blocked by Indo, although the NSAID had no effect on these properties on its own.

CONCLUSIONS

These findings support the hypothesis that PC protects the intestinal mucosa against injurious actions of BS, possibly by forming less toxic mixed micelles. Indo and perhaps other NSAIDs that enter the bile may damage the mucosa, not by a direct action, but by competing for the available protective PC molecules.

Review article: mechanisms of action and therapeutic applications of ursodeoxycholic acid in chronic liver diseases

Ursodeoxycholic acid (ursodiol) is a non-toxic, hydrophilic bile acid used to treat predominantly cholestatic liver disorders. Better understanding of the cellular and molecular mechanisms of action of ursodeoxycholic acid has helped to elucidate its cytoprotective, anti-apoptotic, immunomodulatory and choleretic effects. Ursodeoxycholic acid prolongs survival in primary biliary cirrhosis and it improves biochemical parameters of cholestasis in various other cholestatic disorders including primary sclerosing cholangitis, intrahepatic cholestasis of pregnancy, cystic fibrosis and total parenteral nutrition-induced cholestasis. However, a positive effect on survival remains to be established in these diseases. Ursodeoxycholic acid is of unproven efficacy in non-cholestatic disorders such as acute rejection after liver transplantation, non-alcoholic steatohepatitis, alcoholic liver disease and chronic viral hepatitis. This review outlines the present knowledge of the modes of action of ursodeoxycholic acid, and presents data from clinical trials on its use in chronic liver diseases.

Mechanism of hepatoprotective action of bile salts in liver disease

Ursodeoxycholic acid (UDCA) improves liver enzymes and in many instances liver histology in cholestatic liver diseases such as primary biliary cirrhosis and primary sclerosing cholangitis. Besides classic cholestatic diseases, UDCA also improves liver biochemistry in alcoholic liver disease and in chronic viral hepatitis C. The main target of UDCA treatment, however, is cholestasis, and consequently the mechanisms responsible for the beneficial effects in these diseases are of interest, and are discussed in detail in this article.

The mechanism of biliary lipid secretion and its defects

Biliary lipid secretion is an important physiological event; not only for the disposal of cholesterol from the body, but also for the protection of cells lining the biliary tree against bile salts. Insight into the (patho)physiological role of biliary lipid secretion has been recently expanded through the study of a generation of mice with a disruption of the Mdr2 gene, who do not secrete lipids into bile. Mdr2 P-glycoprotein translocates phospholipids across the hepatocanalicular membrane. These animals suffer from progressive liver disease caused by the toxic detergent action of bile salts. Very recently, it has become clear that an analogous inherited human liver disease exists, which is caused by the absence of biliary lipid secretion. Patients with this disease, Progressive Familial Intrahepatic Cholestasis (PFIC) type 3, have a mutation in the MDR3 gene, which is the human homologue of the murine Mdr2 gene.

Effects of diosgenin, a plant-derived steroid, on bile secretion and hepatocellular cholestasis induced by estrogens in the rat

Increased biliary secretion of cholesterol and lipid vesicles (unilamellae and multilamellae) induced by diosgenin (D), a plant-derived steroid, has cytoprotective effects in the rat liver subjected to obstructive cholestasis. In this study, our aims were to investigate the following: 1) the effects of D on the bile secretory process and on the cholestasis induced by estradiol-17beta-(beta-D-glucuronide) (E17G) or 17 alpha-ethynylestradiol (E) administration; 2) whether the potentially protective effects of D are related to D-induced increase of biliary cholesterol and lipid lamellae; and 3) whether D has other effects capable of modifying specific bile secretory processes or preventing the cholestatic effects of estrogens. Rats were fed a standard ground chow (control group) or chow containing D for 6 days. E17G was administered i.v. to control and D-fed rats and bile flow, bile salt output, and alkaline phosphatase excretion were examined. 17alpha-E was administered from days 4 to 6 to rats fed standard chow or chow plus D for 6 days and different functional parameters of the bile secretory process as well as the ultrastructure of hepatocytes and histochemistry of alkaline phosphatase and Mg2+-adenosine triphosphatase (ATPase) were examined. D-treatment markedly increased cholesterol and lamellar structures in bile and attenuated the acute cholestatic effects of E17G. D-feeding prevented the decrease of taurocholate maximum secretory rate and the increase of biliary alkaline phosphatase and Ca2+,Mg2+-EctoATPase (EctoATPase) excretion, as well as the increase of cholesterol/ phospholipids ratio, alkaline phosphatase activity, and EctoATPase content in canalicular plasma membranes induced by E. D-feeding did not prevent E-induced decrease of basal bile flow, bile salt, cholesterol, and phospholipid secretory rates nor the decrease of Na+,K+-ATPase activity and Na+-taurocholate cotransporting polypeptide (Ntcp) content in isolated sinusoidal membranes. Cholestatic alterations of canalicular domain were apparent in E-treated rats. D administration was also associated with changes of ultrastructure and histochemistry of hepatocytes. E-induced alterations in ultrastructure and acinar distribution and intensity of histochemical reaction of both enzymes were partially prevented by D-feeding. We conclude that D administration, in addition to inducing a marked increase of biliary cholesterol and lipid lamellar structures output, was associated to changes in hepatocyte morphology and plasma membrane composition, enzymes activity, and histochemistry. D-feeding attenuated the acute cholestatic effects of E17G. D-induced increase of bile cholesterol and lipid lamellae content was not apparent when D-fed rats received E. Despite this fact, D administration prevented some cholestatic effects of E, probably through different metabolic effects and/or direct membrane effects, not related to increased lipid lamellae excretion.

Abnormal duodenal bile composition in patients with acalculous chronic cholecystitis

OBJECTIVE

Our goal was to characterize biliary lipid composition in patients with the syndrome of chronic biliary pain, absence of gallstones, and inflammation of the gallbladder mucosa (acalculous chronic cholecystitis).

METHODS

Duodenal bile, obtained from 27 patients with a history of right upper quadrant pain and with negative imaging studies of the biliary tract, was analyzed enzymatically for bile acids, phospholipids, and cholesterol. Fifteen patients were found to have inflammation and/or fibrosis of the gallbladder at cholecystectomy.

RESULTS

The 15 patients with abnormal gallbladder histology had more dilute duodenal bile, as indicated by a low bile acid concentration and a lower proportion of phospholipids (p < 0.01) when values were compared with those of duodenal bile samples from postmenopausal women without gallbladder disease or from radiolucent gallstone subjects participating in the National Cooperative Gallstone Study. Cholecystectomy relieved pain in 9 of 14 patients.

CONCLUSIONS

Some patients with acalculous chronic cholecystitis have duodenal bile samples characterized by a decreased bile acid concentration and a decreased proportion of biliary phospholipids. The low biliary bile acid concentration may result from impaired gallbladder contraction and/or secretion by the biliary tract epithelium. The low proportion of phospholipid may result from posthepatic hydrolysis of luminal phosphatidylcholine followed by absorption of the hydrolysis products. The latter process could be caused by and/or contribute to mucosal inflammation and would also elevate the cholesterol saturation of bile, increasing the risk for cholesterol 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.

Overexpression of mdr2 gene by peroxisome proliferators in the mouse liver

BACKGROUND

In mice, fibrates induce mdr2 gene expression, and its encoded P-glycoprotein in the canalicular domain of hepatocytes, as well as increasing biliary phospholipid output. It is not known whether this effect is restricted to fibrates or is a common property of peroxisome proliferators.

AIMS

To test the effect of structurally unrelated peroxisome proliferators on mdr2 gene expression and biliary phospholipid output, and to explore the molecular mechanism(s) of mdr2 gene induction.

METHODS

Male CFI mice were fed on a diet supplemented with several peroxisome proliferators: phenoxyacetic acid herbicides, plasticizers, acetylsalicylic acid and partially hydrogenated fish oil.

RESULTS

Increased levels of mdr2 mRNAs, assessed by Northern blot analysis, were observed in the liver of mice treated with phenoxyacetic acid herbicides: 2,4,5-trichlorophenoxyacetic acid 570+/-133%, 2,4-dichlorophenoxyacetic acid 233+/-54% (p<0.005); plasticizers: di-(2-ethylhexyl)phthalate 282+/-78%, di-(isoheptyl)phthalate 163+/-40%, phthalic acid dinonyl ester 225+/-48% (p<0.01); and partially hydrogenated fish oil 372+/-138% (p<0.005). P-glycoprotein traffic ATPase content increased in the canalicular domain of hepatocyte of mice treated with the herbicide 2,4,5-trichlorophenoxyacetic acid and with partially hydrogenated fish oil (108% and 87%, respectively, p<0.05) as well as biliary phospholipid output (106% and 74%, respectively, p<0.05). In 2,4,5-trichlorophenoxyacetic acid-fed mice we found five-fold increase on mdr2 transcription rate, assessed by nuclear run-off assay.

CONCLUSIONS

Peroxisome proliferators induce mdr2 gene, its encoded P-gp in the canalicular domain of hepatocytes and increase biliary phospholipid output. The modulation of mdr2 gene might be part of the pleiotrophic response of peroxisome proliferation in mice liver and seems to be regulated mainly at a transcriptional level.

Adaptive regulation of hepatic bile salt transport: role of bile salt hydrophobicity and microtubule-dependent vesicular pathway

BACKGROUND/AIMS

The hepatic transport of bile salts can be regulated by changes in bile salt pool size and/or in the flux of bile salts through the liver. Prolonged bile salt pool depletion is associated with down-regulation of maximum taurocholate transport and decreased canalicular membrane specific bile salt binding sites. This study was undertaken to investigate: a) whether adaptive down-regulation of maximum hepatic bile salt transport occurs to the same extent for bile acids of different hydrophobicity; and b) the role of microtubule-dependent vesicular pathway in the adaptive changes of bile salt transport capacity.

METHODS

Male rats were subjected to 24-h or 48-h external biliary diversion to induce bile salt pool depletion. Basal bile flow, bile salt secretion and lipid secretion, maximum secretory rate of three bile salts of different hydrophobicity (tauroursodeoxycholate, taurocholate and taurochenodeoxycholate) and changes in the biliary excretion of two markers of the microtubule-dependent vesicular pathway (horseradish peroxidase and polyethyleneglycol molecular weight-900) were measured in control and bile salt-depleted rats. Taurocholate-stimulated horseradish peroxidase biliary excretion was also assessed in order to define whether the restoration of bile salt flux across the hepatocytes increased the excretion of this marker in bile salt-depleted rats.

RESULTS

The reduction in the maximum secretory rate of the three bile salts under study observed after prolonged biliary diversion was clearly related to their hydrophobicity, with greater reduction for taurochenodeoxycholate and smaller reduction for tauroursodeoxycholate, compared with taurocholate. The biliary excretion of vesicular transport markers was significantly reduced in bile salt-depleted rats. However, when stimulated by taurocholate, biliary excretion of horseradish peroxidase was similar to controls.

CONCLUSIONS

The magnitude of the decrease of the hepatic bile salt maximum transport capacity seen after bile salt pool depletion is directly related to the hydrophobicity of the bile salt infused. A functionally depressed vesicular transport pathway appears to be also a contributing factor to this phenomenon.

Modulation of intrahepatic cholesterol trafficking: evidence by in vivo antisense treatment for the involvement of sterol carrier protein-2 in newly synthesized cholesterol transport into rat bile

Biliary cholesterol represents one of the two major excretory pathways for sterol elimination from the body and plays a central role in cholesterol gallstone formation. Biliary cholesterol originates from a precursor pool of preformed and newly synthesized free cholesterol. Although it has been suggested that newly synthesized and preformed biliary cholesterol are secreted by independent pathways, the specific cellular and molecular mechanisms are unknown. We used male Wistar rats to study the time-course of the appearance of newly synthesized cholesterol, phosphatidylcholine and protein into bile. The specific role of sterol carrier protein-2 (SCP-2) in the transport of newly synthesized biliary cholesterol was evaluated by an in vivo antisense oligonucleotide approach. In contrast to [14C]phosphatidylcholine and [35S]proteins, the time-course of [14C]cholesterol appearance into bile was rapid, and microtubule- and Golgi-independent. In vivo SCP-2 antisense treatment reduced and delayed the appearance of biliary [14C]cholesterol. Furthermore, hepatic SCP-2 expression increased more than 3-fold over control values in rats that had been treated with diosgenin to increase biliary secretion of newly synthesized cholesterol. These results suggest that SCP-2 is necessary for the rapid transport of newly synthesized cholesterol into bile and that hepatocytes can induce SCP-2 expression according to the rate of biliary secretion of newly synthesized cholesterol.

Fibrates induce mdr2 gene expression and biliary phospholipid secretion in the mouse

Disruption of the murine mdr2 gene leads to the complete absence of biliary phospholipids. We tested the hypothesis that the increase in biliary phospholipid output induced by fibrates is mediated via induction of the hepatic mdr2 gene and its encoded product, the P-glucoprotein canalicular flippase. Increased levels of mdr2 mRNA were observed in the liver of mice treated with different fibrates: ciprofibrate, 660+/-155% (as compared with control group); clofibrate, 611+/-77%; bezafibrate, 410+/-47%; fenofibrate, 310+/-52%; gemfibrozil, 190+/-25% (P <0.05 compared with control group). Induction of expression of the mdr gene family was specific to the mdr2 gene. Two- to three-fold increases in P-glycoprotein immunodetection were evident on the canalicular plasma-membrane domain of clofibrate- and ciprofibrate-treated mice. Biliary phospholipid output increased from 4.2+/-1.2 nmol/min per g of liver in the control group to 8.5+/-0.6, 7.1+/-2.9 and 5.8+/-2.5 in ciprofibrate-, clofibrate- and bezafibrate-treated mice respectively (P <0.05 compared with control group). Moreover, a significant correlation between biliary phospholipid output and the relative levels of mdr2 mRNA was found (r=0.86; P <0.05). In treated animals, bile flow as well as cholesterol and bile acid outputs remained unchanged. Our findings constitute the first evidence that pharmacological modulation of biliary lipid secretion mediated by fibrates can be related to the overexpression of a specific liver gene product, the mdr2 P-glycoprotein, and are consistent with the hypothesis that the mdr2 P-glycoprotein isoform plays a crucial role in the secretion of biliary phospholipid.

Mucus production of choledochal epithelial cells due to bile stagnation in the rat

Mucus secretion is increased by various causative agents of inflammation in the mucosa. Although mucus production of the choledochal epithelial cells (except goblet cells) has never been noted in the rat in normal physiological conditions, it is seen as an inflammatory reaction in animals with choledochoenterostomy, which brings bacteria and active digestive enzymes into the common bile duct through regurgitation of intestinal contents. It is known that stagnant bile alone can cause inflammation in the mucosa of the gallbladder in patients with aseptic acalculous cholecystitis. In this study, aseptic bile stagnation was caused by choledochal dilation made by detaching the common bile duct from the hepatoduodenal ligament in five rats, and histological changes of the choledochal epithelium were observed by light and transmission electron microscopy 17 months after the operative procedure to determine whether stagnant bile could cause mucus production in choledochal epithelial cells. Mucus production was noted in two rats by light microscopy and in all the rats by transmission electron microscopy, and so it was demonstrated that stagnant bile could give rise to this phenomenon in the rat choledochal epithelium.

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.