Chemistry and enterohepatic circulation of bile acids

Bile Acids in the Treatment of Cardiometabolic Diseases

Bile acids (BA), for decades considered only to have fat-emulsifying functions in the gut lumen, have recently emerged as novel cardio-metabolic modulators. They have real endocrine effects, acting via multiple intracellular receptors in various organs and tissues. BA affect energy homeostasis through the modulation of glucose and lipid metabolism, predominantly by activating the nuclear farnesoid X receptor (FXR), as well as the cytoplasmic membrane G protein-coupled BA receptor TGR5 in a variety of tissues; although numerous other intracellular targets of BA are also in play.The roles of BA in the pathogenesis of diabetes, obesity, metabolic syndrome, and cardiovascular diseases are seriously being considered, and BA and their derivatives seem to represent novel potential therapeutics to treat these diseases of civilization.

The Role of the Gut Microbiota in Bile Acid Metabolism

The gut microbiota has been considered a cornerstone of maintaining the health status of its human host because it not only facilitates harvesting of nutrients and energy from ingested food, but also produces numerous metabolites that can regulate host metabolism. One such class of metabolites, the bile acids, are synthesized from cholesterol in the liver and further metabolized by the gut microbiota into secondary bile acids. These bioconversions modulate the signaling properties of bile acids through the nuclear farnesoid X receptor and the G protein-coupled membrane receptor 5, which regulate diverse metabolic pathways in the host. In addition, bile acids can regulate gut microbial composition both directly and indirectly by activation of innate immune response genes in the small intestine. Therefore, host metabolism can be affected by both microbial modifications of bile acids, which leads to altered signaling via bile acid receptors, and by alterations in the composition of the microbiota. In this review, we mainly describe the interactions between bile acids and intestinal microbiota and their roles in regulating host metabolism, but we also examine the impact of bile acid composition in the gut on the intestinal microbiome and on host physiology.

Identification of a mouse Lactobacillus johnsonii strain with deconjugase activity against the FXR antagonist T-β-MCA

Bile salt hydrolase (BSH) activity against the bile acid tauro-beta-muricholic acid (T-β-MCA) was recently reported to mediate host bile acid, glucose, and lipid homeostasis via the farnesoid X receptor (FXR) signaling pathway. An earlier study correlated decreased Lactobacillus abundance in the cecum with increased concentrations of intestinal T-β-MCA, an FXR antagonist. While several studies have characterized BSHs in lactobacilli, deconjugation of T-β-MCA remains poorly characterized among members of this genus, and therefore it was unclear what strain(s) were responsible for this activity. Here, a strain of L. johnsonii with robust BSH activity against T-β-MCA in vitro was isolated from the cecum of a C57BL/6J mouse. A screening assay performed on a collection of 14 Lactobacillus strains from nine different species identified BSH substrate specificity for T-β-MCA only in two of three L. johnsonii strains. Genomic analysis of the two strains with this BSH activity revealed the presence of three bsh genes that are homologous to bsh genes in the previously sequenced human-associated strain L. johnsonii NCC533. Heterologous expression of several bsh genes in E. coli followed by enzymatic assays revealed broad differences in substrate specificity even among closely related bsh homologs, and suggests that the phylogeny of these enzymes does not closely correlate with substrate specificity. Predictive modeling allowed us to propose a potential mechanism driving differences in BSH activity for T-β-MCA in these homologs. Our data suggests that L. johnsonii regulates T-β-MCA levels in the mouse intestinal environment, and that this species may play a central role in FXR signaling in the mouse.

Hepatobiliary transport kinetics of the conjugated bile acid tracer 11C-CSar quantified in healthy humans and patients by positron emission tomography

BACKGROUND & AIMS

Hepatobiliary secretion of bile acids is an important liver function. Here, we quantified the hepatic transport kinetics of conjugated bile acids using the bile acid tracer [N-methyl-¹¹C]cholylsarcosine (¹¹C-CSar) and positron emission tomography (PET).

METHODS

Nine healthy participants and eight patients with varying degrees of cholestasis were examined with ¹¹C-CSar PET and measurement of arterial and hepatic venous blood concentrations of ¹¹C-CSar.

RESULTS

Results are presented as median (range). The hepatic intrinsic clearance was 1.50 (1.20-1.76) ml blood/min/ml liver tissue in healthy participants and 0.46 (0.13-0.91) in patients. In healthy participants, the rate constant for secretion of ¹¹C-CSar from hepatocytes to bile was 0.36 (0.30-0.62)min^-1, 20 times higher than the rate constant for backflux from hepatocytes to blood (0.02, 0.005-0.07min^-1). In the patients, rate constant for transport from hepatocyte to bile was reduced to 0.12 (0.006-0.27)min^-1, 2.3times higher than the rate constant for backflux to blood (0.05, 0.04-0.09). The increased backflux did not fully normalize exposure of the hepatocyte to bile acids as mean hepatocyte residence time of ¹¹C-CSar was 2.5 (1.6-3.1)min in healthy participants and 6.4 (3.1-23.7)min in patients. The rate constant for transport of ¹¹C-CSar from intrahepatic to extrahepatic bile was 0.057 (0.023-0.11)min^-1 in healthy participants and only slightly reduced in patients 0.039 (0.017-0.066).

CONCLUSIONS

This first in vivo quantification of individual steps involved in the hepatobiliary secretion of a conjugated bile acid in humans provided new insight into cholestatic disease.

LAY SUMMARY

Positron emission tomography (PET) using the radiolabelled bile acid (¹¹C-CSar) enabled quantification of the individual steps of the hepatic transport of bile acids from blood to bile in man. Cholestasis reduced uptake and secretion and increased backflux to blood. These findings improve our understanding of cholestatic liver diseases and may support therapeutic decisions.

CLINICAL TRIAL REGISTRATION NUMBER

The trial is registered at ClinicalTrials.gov (NCT01879735).

Association between serum bile acid profiles and gestational diabetes mellitus: A targeted metabolomics study

BACKGROUND

Given the potential influence of aberrant bile acid metabolism on glucose homeostasis, we hypothesized that serum bile acid metabolism is altered in gestational diabetes mellitus (GDM). We characterized the metabolic profiling changes of serum bile acids in GDM and to find the potential biomarkers for the diagnosis and differential diagnosis of GDM.

METHODS

Based on ultrahigh performance liquid chromatography/hybrid quadrupole time-of-flight mass spectrometry, a targeted metabolomics study that involved targeted and untargeted screening techniques was performed to explore the changes in serum bile acid metabolism of GDM cases, intrahepatic cholestasis of pregnancy (ICP) cases and healthy controls.

RESULTS

There were 3 significantly different profiling of serum bile acids for GDM, ICP and controls. Compared to the controls, GDM individuals demonstrated significant increases in 8 bile acid species, including 2 dihydroxy conjugated, 1 trihydroxy unconjugated and 5 sulfated bile acids. β-muricholic acid (β-MCA) and di-2 were well-suited to use as the metabolic markers for the diagnosis and differential diagnosis of GDM, respectively.

CONCLUSIONS

These preliminary findings revealed the protective effect of body against cytotoxicity via elimination of increased sulfated bile acids and aberrant enzyme activity participated in the cycle β-MCA→hyodeoxycholic acid (HDCA) of the bile acid metabolism pathway for the women with GDM, which gave us further insights into the etiology and pathophysiology of GDM.

Novel bile acid therapeutics for the treatment of chronic liver diseases

Recent developments in understanding the role of bile acids (BAs) as signalling molecules in human metabolism and inflammation have opened new avenues in the field of hepatology research. BAs are no longer considered as simple molecules helping in fat digestion but as agents with real therapeutic value in treating complex autoimmune and metabolic liver diseases. BAs and their receptors such as farnesoid X receptor, transmembrane G protein-coupled receptor 5 and peroxisome proliferator-activated receptor have been identified as novel targets for drug development. Some of these novel pharmaceuticals are already in clinical evaluation with the most advanced drugs having reached phase III trials. Chronic liver diseases such as primary biliary cholangitis, primary sclerosing cholangitis and nonalcoholic fatty liver disease, for which there is no or limited pharmacotherapy, are most likely to gain from these developments. In this review we discuss recent and the most relevant basic and clinical research findings related to BAs and their implications for novel therapy for chronic liver diseases.

Activation of Constitutive Androstane Receptor (CAR) in Mice Results in Maintained Biliary Excretion of Bile Acids Despite a Marked Decrease of Bile Acids in Liver

Activation of Constitutive Androstane Receptor (CAR) protects against bile acid (BA)-induced liver injury. This study was performed to determine the effect of CAR activation on bile flow, BA profile, as well as expression of BA synthesis and transport genes. Synthetic CAR ligand 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) was administered to mice for 4 days. BAs were quantified by UPLC-MS/MS (ultraperformance liquid chromatography-tandem mass spectrometry). CAR activation decreases total BAs in livers of male (49%) and female mice (26%), largely attributable to decreases of the 12α-hydroxylated BA taurocholic acid (T-CA) (males (M) 65%, females (F) 45%). Bile flow in both sexes was increased by CAR activation, and the increases were BA-independent. CAR activation did not alter biliary excretion of total BAs, but overall BA composition changed. Excretion of muricholic (6-hydroxylated) BAs was increased in males (101%), and the 12α-OH proportion of biliary BAs was decreased in both males (37%) and females (28%). The decrease of T-CA in livers of males and females correlates with the decreased mRNA of the sterol 12α-hydroxylase Cyp8b1 in males (71%) and females (54%). As a response to restore BAs to physiologic concentrations in liver, mRNA of Cyp7a1 is upregulated following TCPOBOP (males 185%, females 132%). In ilea, mRNA of the negative feedback regulator Fgf15 was unaltered by CAR activation, indicating biliary BA excretion was sufficient to maintain concentrations of total BAs in the small intestine. In summary, the effects of CAR activation on BAs in male and female mice are quite similar, with a marked decrease in the major BA T-CA in the liver.

The role of bile acids in metabolic regulation

Bile acids (BA), long believed to only have lipid-digestive functions, have emerged as novel metabolic modulators. They have important endocrine effects through multiple cytoplasmic as well as nuclear receptors in various organs and tissues. BA affect multiple functions to control energy homeostasis, as well as glucose and lipid metabolism, predominantly by activating the nuclear farnesoid X receptor and the cytoplasmic G protein-coupled BA receptor TGR5 in a variety of tissues. However, BA also are aimed at many other cellular targets in a wide array of organs and cell compartments. Their role in the pathogenesis of diabetes, obesity and other 'diseases of civilization' becomes even more clear. They also interact with the gut microbiome, with important clinical implications, further extending the complexity of their biological functions. Therefore, it is not surprising that BA metabolism is substantially modulated by bariatric surgery, a phenomenon contributing favorably to the therapeutic effects of these surgical procedures. Based on these data, several therapeutic approaches to ameliorate obesity and diabetes have been proposed to affect the cellular targets of BA.

Bile acid malabsorption in inflammatory bowel disease

Bile acid malabsorption (BAM) is a common but an underestimated and often neglected sign of inflammatory bowel diseases (IBDs), especially those affecting the distal ileum. Clinically relevant BAM is most often present in patients with Crohn's ileitis and particularly in ileal-resected Crohn's disease patients. However, deterioration of bile acid (BA) metabolism occurs also in patients with IBD without ileal disease or in those in clinical remission, and the role of BAM in these patients is not well appreciated by clinicians. In a majority of cases, BAM in IBD is caused by impaired conjugated BA reabsorption, mediated by apical sodium/BA cotransporting polypeptide, localized at the luminal surface of the ileal enterocytes. As a consequence, numerous pathological sequelae may occur, including the malfunction of lipid digestion with clinical steatorrhea, impaired intestinal motility, and/or significant changes in the intestinal microflora environment. In this review, a detailed description of the pathophysiological mechanisms of BAM-related diarrhea is presented. Although BAM is present in a significant number of patients with Crohn's disease, its laboratory assessment is not routinely included in diagnostic workups, partially because of costs, logistical reasons, or the unavailability of the more sophisticated laboratory equipment needed. Simultaneously, novel findings related to the effects of the BA signaling pathways on immune functions (mediated through TGR5, cell membrane G protein-coupled BA receptor 1, nuclear farnesoid X receptor, nuclear pregnane X receptor, or nuclear vitamin D receptor) are discussed along with intestinal metabolism in its relationship to the pathogenesis of IBD.

Physiological hypoxia prevents bile salt-induced apoptosis in human and rat hepatocytes

BACKGROUND & AIMS

Hydrophobic bile salts such as glycochenodeoxycholate (GCDC) accumulate in cholestatic liver disease and induce hepatocellular apoptosis, promoting profibrotic signalling. The tissue microenvironment is an integral player in cellular pathophysiology, but it is not routinely incorporated into laboratory studies. Tissue oxygen partial pressure (pO₂) may be an underestimated component of the microenvironment: in the liver, a pO₂ of 30-45 mmHg (approximately 6% O₂) is physiological, because of predominant portal blood supply. It was the aim of this project to investigate the impact of physiological hypoxia (i.e. 6% O₂) on hepatocellular function, namely, bile salt-induced apoptosis.

METHODS

Human hepatoma cells (HepG2-Ntcp) and primary rat hepatocytes were cultured at standard laboratory (hyperoxic) conditions (21% O₂) and at physiological hypoxia (6% O₂) in parallel for 1-8 days to study hepatocellular apoptosis and activation of signalling pathways. Standard laboratory analyses were applied for bile salt uptake, caspase-3/-7 activity, western blotting and gene-array analysis.

RESULTS

Culturing at physiological hypoxia protected both human and rat hepatocytes against GCDC-induced apoptosis: caspase-3/-7 activation was diminished by 3.1 ± 0.5-fold in human HepG2-Ntcp and completely abolished in primary rat hepatocytes. Bile salt uptake was unaffected. Induction of hypoxia-inducible factor-1α indicated adaption to physiological hypoxia. The MEK/ERK cascade was activated and anti-apoptotic mediators were induced: N-Myc down-regulated gene, gelsolin and carbonic anhydrase IX were upregulated 12.4-, 6.5- and 5.2-fold respectively.

CONCLUSIONS

We conclude from these data that (i) physiological hypoxia protects hepatocytes from bile salt-induced apoptosis, (ii) tissue pO₂ is a crucial, underestimated component of the microenvironment and should (iii) be considered when studying hepatocellular physiology in vitro.

Characterization of all RND-type multidrug efflux transporters in Vibrio parahaemolyticus

Resistance nodulation cell division (RND)-type efflux transporters play the main role in intrinsic resistance to various antimicrobial agents in many gram-negative bacteria. Here, we estimated 12 RND-type efflux transporter genes in Vibrio parahaemolyticus. Because VmeAB has already been characterized, we cloned the other 11 RND-type efflux transporter genes and characterized them in Escherichia coli KAM33 cells, a drug hypersusceptible strain. KAM33 expressing either VmeCD, VmeEF, or VmeYZ showed increased minimum inhibitory concentrations (MICs) for several antimicrobial agents. Additional four RND-type transporters were functional as efflux pumps only when co-expressed with VpoC, an outer membrane component in V. parahaemolyticus. Furthermore, VmeCD, VmeEF, and VmeYZ co-expressed with VpoC exhibited a broader substrate specificity and conferred higher resistance than that with TolC of E. coli. Deletion mutants of these transporter genes were constructed in V. parahaemolyticus. TM32 (ΔvmeAB and ΔvmeCD) had significantly decreased MICs for many antimicrobial agents and the number of viable cells after exposure to deoxycholate were markedly reduced. Strains in which 12 operons were all disrupted had very low MICs and much lower fluid accumulation in rabbit ileal loops. These results indicate that resistance nodulation cell division-type efflux transporters contribute not only to intrinsic resistance but also to exerting the virulence of V. parahaemolyticus.

Taurochenodeoxycholic acid induces apoptosis of fibroblast-like synoviocytes

Recent evidences have suggested that the paucity of the apoptosis of fibroblast-like synoviocytes (FLS) may contribute to the pathogenesis of rheumatoid arthritis. Apoptosis induction of rheumatoid arthritis FLS is therefore suggested as a potential therapeutic approach for rheumatoid arthritis. Taurochenodeoxycholic acid (TCDCA), one of the main bioactive substances of animals' bile acid, could favorably ameliorate the progression development and bone destruction of adjuvant arthritis in rat. In this study, we aimed to investigate the possible effect of TCDCA on apoptosis induction of adjuvant arthritis FLS and the mechanisms involved in this process. Apoptosis was determined by flow cytometric analysis. Gene expression levels and the activities of caspase-3 and caspase-8 were evaluated using real time RT-PCR and luminogenic substrates. The activity of nuclear factor-κB (NF-κB) was measured by ELISA. The results showed TCDCA significantly enhanced the apoptosis of adjuvant arthritis FLS in a dose-dependent manner. Besides, TCDCA treatment markedly increased the gene expression level and activity of both caspase-3 and caspase-8. It could suppress the DNA-biding activity of NF-κB. We concluded TCDCA represented an apoptotic effect on adjuvant arthritis FLS via the activation of caspase cascade and this process may be mediated by NF-κB signaling pathway. It was suggested that TCDCA may be a potential therapeutic agent for rheumatoid arthritis.

Ursodeoxycholate modulates bile flow and bile salt pool independently from the cystic fibrosis transmembrane regulator (Cftr) in mice

Cystic fibrosis liver disease (CFLD) is treated with ursodeoxycholate (UDCA). Our aim was to evaluate, in cystic fibrosis transmembrane regulator knockout (Cftr(-/-)) mice and wild-type controls, whether the supposed therapeutic action of UDCA is mediated via choleretic activity or effects on bile salt metabolism. Cftr(-/-) mice and controls, under general anesthesia, were intravenously infused with tauroursodeoxycholate (TUDCA) in increasing dosage or were fed either standard or UDCA-enriched chow (0.5% wt/wt) for 3 wk. Bile flow and bile composition were characterized. In chow-fed mice, we analyzed bile salt synthesis and pool size of cholate (CA). In both Cftr(-/-) and controls intravenous TUDCA stimulated bile flow by ∼250% and dietary UDCA by ∼500%, compared with untreated animals (P < 0.05). In non-UDCA-treated Cftr(-/-) mice, the proportion of CA in bile was higher compared with that in controls (61 ± 4 vs. 46 ± 4%; P < 0.05), accompanied by an increased CA synthesis [16 ± 1 vs. 10 ± 2 μmol·h(-1)·100 g body wt (BW)(-1); P < 0.05] and CA pool size (28 ± 3 vs. 19 ± 1 μmol/100 g BW; P < 0.05). In both Cftr(-/-) and controls, UDCA treatment drastically reduced the proportion of CA in bile below 5% and diminished CA synthesis (2.3 ± 0.3 vs. 2.2 ± 0.4 μmol·day(-1)·100 g BW(-1); nonsignificant) and CA pool size (3.6 ± 0.6 vs. 1.5 ± 0.3 μmol/100 g BW; P < 0.05). Acute TUDCA infusion and chronic UDCA treatment both stimulate bile flow in cystic fibrosis conditions independently from Cftr function. Chronic UDCA treatment reduces the hydrophobicity of the bile salt pool in Cftr(-/-) mice. These results support a potential beneficial effect of UDCA on bile flow and bile salt metabolism in cystic fibrosis conditions.

Bile Acid Inhibition of N-type Calcium Channel Currents from Sympathetic Ganglion Neurons

Under some pathological conditions as bile flow obstruction or liver diseases with the enterohepatic circulation being disrupted, regurgitation of bile acids into the systemic circulation occurs and the plasma level of bile acids increases. Bile acids in circulation may affect the nervous system. We examined this possibility by studying the effects of bile acids on gating of neuronal (N)-type Ca²⁺ channel that is essential for neurotransmitter release at synapses of the peripheral and central nervous system. N-type Ca²⁺ channel currents were recorded from bullfrog sympathetic neuron under a cell-attached mode using 100 mM Ba²⁺ as a charge carrier. Cholic acid (CA, 10^-6 M) that is relatively hydrophilic thus less cytotoxic was included in the pipette solution. CA suppressed the open probability of N-type Ca²⁺ channel, which appeared to be due to an increase in null (no activity) sweeps. For example, the proportion of null sweep in the presence of CA was ~40% at +40 mV as compared with ~8% in the control recorded without CA. Other single channel properties including slope conductance, single channel current amplitude, open and shut times were not significantly affected by CA being present. The results suggest that CA could modulate N-type Ca²⁺ channel gating at a concentration as low as 10^-6 M. Bile acids have been shown to activate nonselective cation conductance and depolarize the cell membrane. Under pathological conditions with increased circulating bile acids, CA suppression of N-type Ca²⁺ channel function may be beneficial against overexcitation of the synapses.

Cytoprotective properties of rifampicin are related to the regulation of detoxification system and bile acid transporter expression during hepatocellular injury induced by hydrophobic bile acids

BACKGROUND/PURPOSE

Rifampicin has been used for the treatment of patients with jaundice and pruritus. This study evaluated the effect of rifampicin on the expression of different detoxification systems and bile acid transporters during in-vivo and in-vitro experimental models of cholestasis.

METHODS

Rifampicin was administered to glycochenodeoxycholic acid (GCDCA)-treated human hepatocytes and bile duct-obstructed rats. Different parameters related to cell death, and the expression of phase I and II drug metabolizing enzymes (DME) and bile acid transporters were determined.

RESULTS

The induction of hepatocellular injury induced by cholestasis was associated with a reduction in cytochrome P4503A4 (CYP3A4), CYP7A1, and UDP-glucuronosyltransferase 2B4 (UGT2B4) expression, as well as an increase in import (Na(+)-taurocholate co-transporting polypeptide, NTCP) system expression. The beneficial properties of rifampicin were associated with an increase in DME and export bile acid systems (multidrug resistance-associated protein 4, MRP4, and bile acid export pump to bile duct, BSEP) expression, as well as a reduction in NTCP expression.

CONCLUSIONS

The beneficial effect of rifampicin in cholestasis is associated with an increase in DME expression involved in toxic, bile acid and cholesterol metabolism, as well as a reduction in the bile acid importing system in hepatocytes.

Effects of taurochenodeoxycholic acid on adjuvant arthritis in rats

Taurochenodeoxycholic acid (TCDCA) is one of the main bioactive substances of animals' bile acid. In this study, we aimed to investigate the anti-arthritic effects and potential mechanism of TCDCA on adjuvant arthritis (AA) in rats. Freund's complete adjuvant (FCA) was used to induce AA in rats. Paw swelling, index of thymus and spleen and body weight growth rate were measured, and polyarthritis index and radiologic changes were observed. The production of TNF-α, IL-1β, IL-6 and IL-10 was detected by ELISA in serum and synoviocytes. mRNA expression of TNF-α, IL-1β, IL-6 and IL-10 was determined by real-time RT-PCR in synovium tissue and synoviocytes. In both prophylactic and therapeutic treatment, TCDCA significantly suppressed paw swelling and polyarthritis index, increased the loss body weight and index of thymus and spleen, and amended radiologic changes in AA rats. The overproduction and mRNA expression of TNF-α, IL-1β and IL-6 were remarkably suppressed in serum and synovium tissue of all TCDCA-treated rats, however, IL-10 was markedly increased in prophylactic treatment. In a definite concentration ranging from 300 μg/mL to 500 μg/mL, TCDCA showed marked inhibition in the overproduction and mRNA expression of TNF-α, IL-1β and IL-6 in synoviocytes in a concentration-dependent manner, but opposite action on IL-10. In conclusion, treatment with TCDCA confers a good anti-adjuvant arthritis activity in rats, which its reparative effects could be mediated via reduction of the protein and mRNA expression of TNF-α, IL-1β and IL-6, and augment of IL-10 in rats.

Nitric oxide mimics transcriptional and post-translational regulation during α-tocopherol cytoprotection against glycochenodeoxycholate-induced cell death in hepatocytes

BACKGROUND & AIMS

Reactive oxygen species (ROS) and nitric oxide (NO) exert a relevant role during bile acid-induced hepatotoxicity. Whether α-Tocopherol regulates oxidative and nitrosative stress, bile acid transporter expression and their NO-dependent post-translational modifications, and cell death were assessed in vitro and in vivo.

METHODS

α-Tocopherol and/or NO donors (DETA-NONOate or CSNO, and V-PYRRO/NO) were administered to glycochenodeoxycholic acid (GCDCA)-treated cultured human hepatocytes or to bile duct obstructed rats. Cell injury, superoxide anion (O⁻₂) production, as well as inducible nitric oxide synthase (NOS-2), cytochrome P4507A1 (CYP7A1), heme oxygenase-1, (HO-1) and bile acid transporter expression were determined. Cysteine S-nitrosylation and tyrosine nitration of Na(+)-taurocholate co-transporting polypeptide (NTCP), as well as taurocholic acid (TC) uptake were also evaluated.

RESULTS

GCDCA-induced cell death was associated with increased (O⁻₂) production, NTCP and HO-1 expression, and with a reduction of CYP7A1 and NOS-2 expression. α-Tocopherol reduced cell death, (O⁻₂) production, CYP7A1, NTCP, and HO-1 expression, as well as increased NOS-2 expression and NO production in GCDCA-treated hepatocytes. α-Tocopherol and NO donors increased NTCP cysteine S-nitrosylation and tyrosine nitration, and reduced TC uptake in hepatocytes. α-Tocopherol and V-PYRRO/NO reduced liver injury and NTCP expression in obstructed rats.

CONCLUSIONS

The regulation of CYP7A1, NTCP, and HO-1 expression may be relevant for the cytoprotective properties of α-Tocopherol and NO against mitochondrial dysfunction, oxidative stress and cell death in GCDCA-treated hepatocytes. The regulation of NO-dependent post-translational modifications of NTCP by α-Tocopherol and NO donors reduces the uptake of toxic bile acids by hepatocytes.

Mitochondrial-driven ubiquinone enhances extracellular calcium-dependent nitric oxide production and reduces glycochenodeoxycholic acid-induced cell death in hepatocytes

Ca(2+) mobilization, nitric oxide (NO), and oxidative stress have been involved in cell death induced by hydrophobic bile acid in hepatocytes. The aim of the study was the elucidation of the effect of the antioxidant mitochondrial-driven ubiquinone (Mito Q) on the intracellular Ca(2+) concentration, NO production, and cell death in glycochenodeoxycholic acid (GCDCA)-treated HepG2 cells. The role of the regulation of the intracellular Ca(2+) concentration by Ca(2+) chelators (EGTA or BAPTA-AM), agonist of Ca(2+) entrance (A23187) or NO (L-NAME or NO donor), was assessed during Mito Q cytoprotection in GCDCA-treated HepG2 cells. Cell death, NO synthase (NOS)-1, -2, and -3 expression, Ca(2+) mobilization, and NO production were evaluated. GCDCA reduced the intracellular Ca(2+) concentration and NOS-3 expression and enhanced cell death in HepG2. NO donor prevented and L-NAME enhanced GCDCA-induced cell death. The reduction of Ca(2+) entry by EGTA, but not its release from intracellular stores by BAPTA-AM, reduced the expression of NOS-3 and enhanced cell death in control and GCDCA-treated cells. Mito Q prevented the reduction of intracellular Ca(2+) concentration, NOS-3 expression, NO production, and cell death in GCDCA-treated HepG2 cells. The conclusion is that the recovery of Ca(2+)-dependent NOS-3 expression by Mito Q may be considered an additional cytoprotective property of an antioxidant.

Apoptosis-inducing effect of synthetic chenodeoxycholic acid derivative, HS-1200, in human hepatoma cell line BEL-7402 and the mechanisms involved

Chenodeoxycholic acid (CDCA) derivative HS-1200 can inhibit the proliferation and induce the apoptosis of human hepatoma BEL7402 cells in a dose- and time-dependent manner. In contrast, HS-1200 shows no apoptosis-inducing effect in normal human hepatic cell lines. HS-1200 induces the apoptosis of BEL7402 cells perhaps by up-regulating the expression of Bax protein and down-regulating the expression of Bcl-2 protein. The increased ratio of Bax to Bcl-2 might contribute to the permeabilization of the outer mitochondrial membrane (OMM) and make it permeable to intermembrane space proteins such as cytochrome C. Once released, cytochrome C promotes the activation of caspase-9 and thereby results in the activation of caspase-3, which functions as the downstream effector of the cell death program. Furthermore, as caspase-8-specific inhibitor z-IETDfmk shows no impact on HS-1200-mediated apoptosis of BEL7402 cells, HS-1200 induces apoptosis perhaps via the activation of mitochondrial pathway.

Regulation of the proprotein convertase subtilisin/kexin type 9 in intestinal epithelial cells

Proprotein convertase subtilisin/kexin type 9 (PCSK9) posttranslationally promotes the degradation of the low-density lipoprotein receptor (LDLr) in hepatocytes and increases plasma LDL cholesterol. It is not clear, however, whether PCSK9 plays a role in the small intestine. Here, we characterized the patterns of variations of PCSK9 and LDLr in fully differentiated Caco-2/15 cells as a function of various potential effectors. Cholesterol (100 microM) solubilized in albumin or micelles significantly downregulated PCSK9 gene (30%, P<0.05) and protein expression (50%, P<0.05), surprisingly in concert with a decrease in LDLr protein levels (45%, P<0.05). Cells treated with 25-hydroxycholesterol (50 microM) also displayed significant reduction in PCSK9 gene (37%, P<0.01) and protein (75% P<0.001) expression, whereas LDLr showed a decrease at the gene (30%, P<0.05) and protein (57%, P<0.01) levels, respectively. The amounts of PCSK9 mRNA and protein in Caco-2/15 cells were associated to the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and sterol regulatory element binding protein-2 (SREBP-2) that can transcriptionally activate PCSK9 via sterol-regulatory elements located in its proximal promoter region. On the other hand, depletion of cholesterol content by hydroxypropyl-beta-cyclodextrin upregulated PCSK9 transcripts (20%, P<0.05) and protein mass (540%, P<0.001), in parallel with SREBP-2 protein levels. The addition of bile acids (BA) taurocholate and deoxycholate to the apical culture medium lowered PCSK9 gene expression (25%, P<0.01) and raised PCSK9 protein expression (30%, P<0.01), respectively, probably via the modulation of farnesoid X receptor. Furthermore, unconjugated and conjugated BA exhibited different effects on PCSK9 and LDLr. Altogether, these data indicate that intestinal PCSK9 is highly modulated by sterols and emphasize the distinct effects of BA species.

Bile acids and signal transduction: role in glucose homeostasis

Bile acids are mainly recognized for their role in dietary lipid absorption and cholesterol homeostasis. However, recent progress in bile acid research suggests that bile acids are important signaling molecules that play a role in glucose homeostasis. Among the various supporting evidence, several reports have demonstrated an improvement of the glycemic index of type 2 diabetic patients treated with diverse bile acid binding resins. Herein, we review the diverse interactions of bile acids with various signaling/response pathways, including calcium mobilization and protein kinase activation, membrane receptor-mediated responses, and nuclear receptor responses. Some of the effects of the bile acids are direct through the activation of specific receptors, i.e., TGR5, CAR, VDR, and FXR, while others imply modulation of the hormonal, growth factor and/or neuromediator responses, i.e., glucagon, EGF, and acetylcholine. We also discuss recent evidence implicating the interaction of bile acids with glucose homeostasis mechanisms, with the integration of our understanding of how the signaling mechanisms modulated by bile acid could regulate glucose metabolism.

Synthetic chenodeoxycholic acid derivative, HS-1200, induces apoptosis of human hepatoma cells via a mitochondrial pathway

We investigated whether HS-1200 has anti-proliferation effects on human hepatoma cells in vitro. Here, chromatin condensation, DNA ladder formation and proteolytic cleavage of poly (ADP-ribose) polymerase (PARP) were observed after treatment of HS-1200, indicating the occurrence of apoptotic cell death, which was associated with up-regulation of Bax, cleaved-caspase-3 and cleaved-caspase-9. Inhibition of caspase-9 rescued HS-1200-induced apoptosis. Furthermore, cells treated with HS-1200 showed a reduction in mitochondrial membrane potential (Deltapsi(m)) and caused cytochrome c release into the cytosol. The results indicated that synthetic chenodeoxycholic acid HS-1200 could induce cell apoptosis in BEL7402 human hepatoma cell line, via a Bax/cytochrome c/caspase-9 independent pathway. This study suggested that HS-1200 is potentially useful as an apoptosis inducer for the treatment of hepatocellular carcinoma.

A chenodeoxycholic derivative, HS-1200, induces apoptosis and cell cycle modulation via Egr-1 gene expression control on human hepatoma cells

We previously reported that HS-1200, a synthetic chenodeoxycholic acid derivative, has apoptosis-inducing activity in various human cancer cells. The present study was undertaken to examine whether HS-1200 had an anticancer effect on HepG2 (wild-type p53) and Hep3B (p53 deleted) human hepatoma cells. Treatment of both cells with HS-1200 resulted in growth inhibition and induction of apoptosis as measured by MTT assay, nuclear staining, DNA fragmentation and flow cytometry analysis. The increase in apoptosis was associated with the alteration in the ratio of Bcl-2/Bax protein expression. In addition, flow cytometry analysis indicated that HS-1200 induced G1 phase arrest in both cells. When analyzing the expression of cell cycle-related proteins, we found that HS-1200 reduced the expression levels of cyclin D1, cyclin A, and Cdk2. HS-1200 treatment also caused an increase in the expression levels of p21 WAF1/CIP1 in HepG2 cells in a p53-dependent manner and in Hep3B cells in a p53-independent manner. Moreover, the expression level of p27 KIP1 was increased in both cell lines. We also observed that HS-1200 decreased the levels of cyclooxygenase (COX)-2 mRNA and protein expression. Furthermore, HS-1200 treatment markedly induced the Egr-1 expression at an early time point, and the increased expression levels of p53, p21 WAF1/CIP1, p27 KIP1, and COX-2 after treatment with HS-1200 were completely inhibited in HepG2 cells and partially inhibited in Hep3B cells by silencing of Egr-1, respectively. Taken together, these findings provide important new insights into the possible molecular mechanisms of the anticancer activity of the synthetic bile acid derivative, HS-1200, through Egr-1 regulation.

Bile acids and Barrett's oesophagus: a sine qua non or coincidence?

BACKGROUND

Barrett's oesophagus (BO), a premalignant condition associated with the development of oesophageal adenocarcinoma (OAC), is thought to be a consequence of chronic duodeno-gastro-oesophageal reflux. Of the refluxates, bile acids, either alone or in combination with acid, are probably the most important.

METHODS

Analysis of the literature on the role played by bile acids in inducing BO and/or progression to OAC.

RESULTS

Combined pH and Bilitec 2000 (as a measure of bile reflux) monitoring and oesophageal aspiration studies in humans suggest a combined role for bile acids, particularly taurine conjugated bile acids, in causing oesophageal mucosal injury. Evidence from animal models has demonstrated that duodenal juice alone is also able to induce BO and/or OAC. Likewise, ex vivo studies with biopsies from BO patients show that increased proliferation and cyclo-oxygenase-2 expression are present after a pulsed exposure to acid or conjugated bile acids, but not if acid and bile acids are combined. Proton-pump inhibitors (PPIs) have been shown to decrease the biliary component of the refluxate. There is some evidence that PPIs are able to reduce neoplastic progression in BO. On the other hand, chronic PPIs can also stimulate bacterial overgrowth, which can result in increased production of secondary bile acids, particularly deoxycholic acid, in the stomach. Deoxycholic acid has been demonstrated to have a tumour-promoting capacity.

CONCLUSIONS

It is unknown what factors of the refluxate (acid and/or bile) induce BO and/or promote carcinogenesis, but there is evidence that secondary bile acids play a role. A better understanding of the molecular steps involved in the induction of BO, and the role of bile acids herein, may identify targets at which preventive therapies can be directed.

3α-6α-Dihydroxy-7α-fluoro-5β-cholanoate (UPF-680), physicochemical and physiological properties of a new fluorinated bile acid that prevents 17α-ethynyl-estradiol-induced cholestasis in rats

3alpha-6alpha-Dihydroxy-7alpha-fluoro-5beta-cholanoate (UPF-680), the 7alpha-fluorine analog of hyodeoxycholic acid (HDCA), was synthesized to improve bioavailability and stability of ursodeoxycholic acid (UDCA). Acute rat biliary fistula and chronic cholestasis induced by 17alpha-ethynyl-estradiol (17EE) models were used to study and compare the effects of UPF-680 (dose range 0.6-6.0 micromol/kg min) with UDCA on bile flow, biliary bicarbonate (HCO(3)(-)), lipid output, biliary bile acid composition, hepatic enzymes and organic anion pumps. In acute infusion, UPF-680 increased bile flow in a dose-related manner, by up to 40.9%. Biliary HCO(3)(-) output was similarly increased. Changes were observed in phospholipid secretion only at the highest doses. Treatment with UDCA and UPF-680 reversed chronic cholestasis induced by 17EE; in this model, UDCA had no effect on bile flow in contrast to UPF-680, which significantly increased bile flow. With acute administration of UPF-680, the biliary bile acid pool became enriched with unconjugated and conjugated UPF-680 (71.7%) at the expense of endogenous cholic acid and muricholic isomers. With chronic administration of UPF-680 or UDCA, the main biliary bile acids were tauro conjugates, but modification of biliary bile acid pool was greater with UPF-680. UPF-680 increased the mRNA for cytochrome P450 7A1 (CYP7A1) and cytochrome P450 8B (CYP8B). Both UDCA and UPF-680 increased the mRNA for Na(+) taurocholate co-transporting polypeptide (NCTP). In conclusion, UPF-680 prevented 17EE-induced cholestasis and enriched the biliary bile acid pool with less detergent and cytotoxic bile acids. This novel fluorinated bile acid may have potential in the treatment of cholestatic liver disease.

Mice deleted for fatty acid transport protein 5 have defective bile acid conjugation and are protected from obesity

BACKGROUND & AIMS

Fatty Acid Transport Protein 5 (FATP5) is a liver-specific member of the FATP/Slc27 family, which has been shown to exhibit both fatty acid transport and bile acid-CoA ligase activity in vitro. Here, we investigate its role in bile acid metabolism and body weight homeostasis in vivo by using a novel FATP5 knockout mouse model.

METHODS

Bile acid composition was analyzed by mass spectroscopy. Body weight, food intake, energy expenditure, and fat absorption were determined in animals fed either a low- or a high-fat diet.

RESULTS

Although total bile acid concentrations were unchanged in bile, liver, urine, and feces of FATP5 knockout mice, the majority of gallbladder bile acids was unconjugated, and only a small percentage was conjugated. Primary, but not secondary, bile acids were detected among the remaining conjugated forms in FATP5 deletion mice, suggesting a specific requirement for FATP5 in reconjugation of bile acids during the enterohepatic recirculation. Fat absorption in FATP5 deletion mice was largely normal, and only a small increase in fecal fat was observed on a high-fat diet. Despite normal fat absorption, FATP5 deletion mice failed to gain weight on a high-fat diet because of both decreased food intake and increased energy expenditure.

CONCLUSIONS

Our findings reveal an important role for FATP5 in bile acid conjugation in vivo and an unexpected function in body weight homeostasis, which will require further analysis. FATP5 deletion mice provide a new model to study the intersection of bile acid metabolism, lipid metabolism, and body weight regulation.

Phosphatidylinositol 3-kinase-dependent signaling modulates taurochenodeoxycholic acid-induced liver injury and cholestasis in perfused rat livers

Taurochenodeoxycholic acid (TCDCA), but not glycochenodeoxycholic acid (GCDCA), activates a phosphatidylinositol 3-kinase (PI3-K)-mediated survival pathway in vitro. Here, the effects of PI3-K inhibition on TCDCA- and GCDCA-induced hepatocellular injury, apoptosis, and bile secretion were examined in the intact liver. In isolated perfused rat livers, bile flow was determined gravimetrically. Hepatovenous lactate dehydrogenase and alanine aminotransferase efflux as markers of liver integrity and biliary secretion of 2,4-dinitrophenyl-S-glutathione (DNP-GS) were determined photometrically. Apoptosis was assessed by immunohistochemistry of active caspase-3 and cytokeratin 18 in liver tissue. Phosphorylation of protein kinase B (PKB/Akt) as a readout of PI3-K activity was determined by immunoblot analysis. Bile acid concentrations were determined by gas chromatography. TCDCA (25 muM) induced moderate liver injury by hepatocellular apoptosis and distinctly reduced bile flow and DNP-GS secretion. In contrast, GCDCA (25 muM) induced severe liver injury by extensive hepatocyte apoptosis. TCDCA strongly activated PI3-K, whereas GCDCA did not markedly affect PI3-K activity. Inhibition of PI3-K by 100 nM wortmannin enhanced TCDCA-induced liver injury and apoptosis and tended to aggravate the cholestatic effect of TCDCA. In contrast, wortmannin reduced GCDCA-induced liver injury and apoptosis. Bile acid uptake tended to be reduced by wortmannin. The cholestatic effect of GCDCA was aggravated by wortmannin. Inhibition of PI3-K markedly aggravated TCDCA-induced but not GCDCA-induced liver damage and hepatocyte apoptosis. Thus TCDCA appears to block its inherent toxicity by a PI3-K-dependent survival pathway in the intact liver.

Effect of the adaptation to high bile salts concentrations on glycosidic activity, survival at low PH and cross-resistance to bile salts in Bifidobacterium

Six derivatives with increased resistance to ox gall (MIC: > or = 1% w/v) and one derivative resistant to sodium cholate (MIC: 0.8% w/v) were obtained from more sensitive original Bifidobacterium strains. These microorganisms, and two additional cholate resistant derivatives obtained in a previous study (Int. J. Food Microbiol. 82 (2003) 191), were partially characterised in this study. Acquisition of resistance against a given bile salt, also conferred cross-resistance to other bile salts, and promoted an increase in the survival of these microorganisms at low pH. Bile resistance levels of derivatives were dependent on the external pH so that the resistance was lower at neutral pH values than in acidic environments. In addition, the acquisition of bile resistance induced changes on glycoside-hydrolysing activities of derivatives obtained from five out of eight original strains, with certain activities such as beta-glucosidase showing more than tenfold increases in some of these microorganisms. These data suggest that the exposure to high bile salts concentrations may have induced a synergic response on Bifidobacterium for the adaptation to the conditions of the gastrointestinal tract. This could have improved the survival at low pH in these microorganisms, the resistance to high bile salts concentrations, and the assimilation of non-digestible carbohydrates by the enhancement of some glycoside-hydrolysing activities.

Apoptosis and modulation of cell cycle control by synthetic derivatives of ursodeoxycholic acid and chenodeoxycholic acid in human prostate cancer cells

The effects of synthetic derivatives of ursodeoxycholic acid (UDCA), HS-1183, and chenodeoxycholic acid (CDCA), HS-1199 and HS-1200, on the proliferation of human prostate carcinoma PC-3 cells were investigated. Whereas CDCA and UDCA had no effects on the growth of cells in a concentration range we have tested, HS-1199 and HS-1200 completely inhibited the cell proliferation, and HS-1183 showed a weak inhibitory activity. This proliferation-inhibitory effect of the synthetic bile acid derivatives was due to the induction of apoptosis, which was confirmed by observing DNA fragmentation, chromatin condensation and cleavage of PARP. Flow cytometric analysis also revealed that the synthetic bile acid derivatives arrested the cell cycle progression at the G1 phase, which effects were associated with inhibition of phosphorylation of pRB and enhanced binding of pRB and E2F-1. They also suppressed Cdk2 and cyclin E-dependent kinase activities without changes of their expressions. Furthermore, the synthetic bile acids increased the levels of Cdk inhibitor, p21WAF1/CIP1, expression and activated the reporter construct of p21WAF1/CIP1 promoter in p53-independent manner, and p21WAF1/CIP1 proteins induced by the synthetic bile acid derivatives were associated with Cdk2 and proliferating cell nuclear antigen. These distinctive features suggest that it is possible to create the new drugs useful for cancer therapy from the synthetic bile acid derivatives as lead compounds.

Liver disease caused by failure to racemize trihydroxycholestanoic acid: gene mutation and effect of bile acid therapy

BACKGROUND & AIMS

Inborn errors of bile acid metabolism may present as neonatal cholestasis and fat-soluble vitamin malabsorption or as late onset chronic liver disease. Our aim was to fully characterize a defect in bile acid synthesis in a 2-week-old African-American girl presenting with coagulopathy, vitamin D and E deficiencies, and mild cholestasis and in her sibling, whose liver had been used for orthotopic liver transplantation (OLT).

METHODS

Bile acids were measured by mass spectrometry in urine, bile, serum, and feces of the patient and in urine from the unrelated recipient.

RESULTS

Liver biopsy specimens showed neonatal hepatitis with giant cell transformation and hepatocyte necrosis; peroxisomes were reduced in number. High concentrations of (25R)3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid in the urine, bile, and serum established a pattern similar to that of Zellweger syndrome and identical to the Alligator mississippiensis. Serum phytanic acid was normal, whereas pristanic acid was markedly elevated. Biochemical, MRI, and neurologic findings were inconsistent with a generalized defect of peroxisomal function and were unique. Analysis of the urine from the recipient of the deceased sibling's liver confirmed the same bile acid synthetic defect. A deficiency in 2-methylacyl-CoA racemase, which is essential for conversion of (25R)THCA to its 25S-isomer, the substrate to initiate peroxisomal beta-oxidation to primary bile acids, was confirmed by DNA analysis revealing a missense mutation (S52P) in the gene encoding this enzyme. Long-term treatment with cholic acid normalized liver enzymes and prevented progression of symptoms.

CONCLUSIONS

This genetic defect further highlights bile acid synthetic defects as a cause of neonatal cholestasis.

Diet and cancer: assessing the risk

Globally, colorectal cancer (CRC) is a leading cause of mortality from malignant disease. Case-control and cohort studies provide strong support for a role of diet in the aetiology of CRC. However to establish causal relationships and to identify more precisely the dietary components involved, intervention studies in human subjects are required. Cancer is an impractical endpoint in terms of numbers, cost, study duration and ethical considerations. Consequently, intermediate biomarkers of the disease are required. This review aims to provide an overview of the intermediate endpoints available for the study of CRC, particularly non-invasive faecal biomarkers. Examples of their use in dietary intervention studies are given.

Bile salts, intestinal microflora and enterohepatic circulation

Use of probiotic therapy implies an attempt to maintain the normal bacterial flora or to alter the bacterial flora so that organisms beneficial to the host are nurtured. We also know that the bacterial flora are essential in the deconjugation and transformation of bile acid metabolism. The relationships of the bile acids to intestinal bacteria and their recurrent exposure in the enterohepatic circulation are discussed.

Interactions of combined bile acids on hepatocyte viability: cytoprotection or synergism

Cholestasis results from hepatocyte dysfunction due to the accumulation of bile acids in the cell, many of which are known to be cytotoxic. Recent evidence implicates competitive antagonism of key cytotoxic responses as the mechanism by which certain therapeutic bile acids might afford cytoprotection against cholestasis. In this work, we compare the relative cytotoxicity of bile acids in terms of dose- and time-dependence. To better elucidate the controversy related to the therapeutic use of ursodeoxycholate (UDCA) in cholestatic patients, we also evaluated the effects of bile acid combinations. Viability of Wistar rat hepatocytes in primary culture was measured by LDH leakage after 12 and 24 h exposure of cells to the various bile acids. All unconjugated bile acids caused a dose-dependent decrease in cell viability. The tauro- and glyco-conjugates of chenodeoxycholate (CDCA) and UDCA were all less toxic than the corresponding unconjugated form. Although relatively non-toxic, UDCA caused synergistic cell killing by lithocholate (LCA), CDCA, glyco-CDCA (GCDC) and tauro-CDCA (TCDC). Glycoursodeoxycholate decreased the toxicity of GCDC, but potentiated the toxicity of unconjugated CDCA and LCA. The tauro-conjugate of UDCA had no significant effect. These data suggest that at cholestatic concentrations, bile acid-induced cell death correlates with the degree of lipophilicity of individual bile acids. However, these results indicate that the reported improvement of biochemical parameters in cholestatic patients treated with UDCA is not due to a direct effect of UDCA on hepatocyte viability. Therefore, any therapeutic effect of UDCA must be secondary to some other process, such as altered membrane transport or nonparenchymal cell function.

Cholic acid is accumulated spontaneously, driven by membrane deltapH, in many lactobacilli

Many lactobacilli from various origins were found to apparently lack cholic acid extrusion activity. Cholic acid was accumulated spontaneously, driven by the transmembrane proton gradient. Accumulation is a newly identified kind of interaction between intestinal microbes and unconjugated bile acids and is different from extrusion and modification, which have been described previously.

Cholate resistance in Lactococcus lactis is mediated by an ATP-dependent multispecific organic anion transporter

The cholate-resistant Lactococcus lactis strain C41-2, derived from wild-type L. lactis MG1363 through selection for growth on cholate-containing medium, displayed a reduced accumulation of cholate due to an enhanced active efflux. However, L. lactis C41-2 was not cross resistant to deoxycholate or cationic drugs, such as ethidium and rhodamine 6G, which are typical substrates of the multidrug transporters LmrP and LmrA in L. lactis MG1363. The cholate efflux activity in L. lactis C41-2 was not affected by the presence of valinomycin plus nigericin, which dissipated the proton motive force. In contrast, cholate efflux in L. lactis C41-2 was inhibited by ortho-vanadate, an inhibitor of P-type ATPases and ATP-binding cassette transporters. Besides ATP-dependent drug extrusion by LmrA, two other ATP-dependent efflux activities have previously been detected in L. lactis, one for the artificial pH probe 2',7'-bis-(2-carboxyethyl)-5(and 6)-carboxyfluorescein (BCECF) and the other for the artificial pH probe N-(fluorescein thio-ureanyl)-glutamate (FTUG). Surprisingly, the efflux rate of BCECF, but not that of FTUG, was significantly enhanced in L. lactis C41-2. Further experiments with L. lactis C41-2 cells and inside out membrane vesicles revealed that cholate and BCECF inhibit the transport of each other. These data demonstrate the role of an ATP-dependent multispecific organic anion transporter in cholate resistance in L. lactis.

The bile acid taurochenodeoxycholate activates a phosphatidylinositol 3-kinase-dependent survival signaling cascade

Liver injury during cholestasis reflects a balance between the effects of toxic and nontoxic bile acids. However, the critical distinction between a toxic and nontoxic bile acid remains subtle and unclear. For example, the glycine conjugate of chenodeoxycholate (GCDC) induces hepatocyte apoptosis, whereas the taurine conjugate (TCDC) does not. We hypothesized that the dissimilar cellular responses may reflect differential activation of a phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathway. In the bile acid-transporting McNtcp.24 rat hepatoma cell line, TCDC, but not GCDC, stimulated PI3K activity. Consistent with this observation, inhibition of PI3K rendered TCDC cytotoxic, and constitutive activation of PI3K rendered GCDC nontoxic. Both Akt and the atypical protein kinase C isoform zeta (PKCzeta) have been implicated in PI3K-dependent survival signaling. However, TCDC activated PKCzeta, but not Akt. Moreover, inhibition of PKCzeta converted TCDC into a cytotoxic agent, whereas overexpression of wild-type PKCzeta blocked GCDC-induced apoptosis. We also demonstrate that TCDC activated nuclear factor kappaB (NF-kappaB) in a PI3K- and PKCzeta-dependent manner. Moreover, inhibition of NF-kappaB by an IkappaB super-repressor rendered TCDC cytotoxic, suggesting that NF-kappaB is also necessary to prevent the cytotoxic effects of TCDC. Collectively, these data suggest that some hydrophobic bile acids such as TCDC activate PI3K-dependent survival pathways, which prevent their otherwise inherent toxicity.

Ileal absorption of bile acids in patients with chronic cholestasis: SeHCAT test results and effect of ursodeoxycholic acid (UDCA)

The effect of cholestasis on ileal bile acid absorption is controversial in animal models (up- or down-regulation) and unknown in humans. We therefore studied values of the selena homotaurocholic acid (SeHCAT) test before and after long-term administration (>3 months, 13-15 mg/kg/day) of ursodeoxycholic acid (UDCA) in 27 patients with chronic cholestatic liver diseases (24 women, 3 men; mean age, 50 years; 24 primary biliary cirrhosis, 2 secondary biliary cirrhosis, 2 others). The control group consisted of 14 healthy volunteers. Seven-day SeHCAT percentage retention was identical in the 12 untreated cholestatic patients (serum bilirubin, 75+/-42 micromol/L, alkaline phosphatase, 4.2+/-1.0 N; mean+/-SEM) and in the control group (43.6+/-2.9 and 43.8+/-4.2%, respectively). In the 22 patients treated by UDCA for 38+/-8 months, SeHCAT percentage retention was 20.3+/-3.0%. In the seven patients with the SeHCAT test done before and after UDCA treatment (16+/-5 months), SeHCAT percentage retention decreased significantly under UDCA therapy (42.0+/-4.4 vs 19.4+/-4.1%; P < 0.02). We conclude that, in patients with chronic cholestasis (1) SeHCAT percentage retention is not altered-taken together with the known defect of biliary excretion, this lack of increase in SeHCAT percentage retention argues against up-regulation of bile acid ileal transport; and (2) UDCA treatment induces a decrease in the SeHCAT percentage retention-this effect may be related primarily to a decreased bile acid ileal absorption.

Effects of cholestyramine and parenteral nutrition on hepatic metabolism of lidocaine: a study using isolated rat liver perfusion

BACKGROUND

The effect of an oral bile salt binder, cholestyramine, on parenteral nutrition-related hepatic dysfunction and lidocaine metabolism was studied in rats.

METHODS

Rats were randomly assigned to one of three treatment groups: the PN group received infusions of dextrose and amino acids; the PNC group was treated the same as the PN group, but also received oral cholestyramine; CF group animals were fed rat food and water. Lidocaine metabolism was studied in livers isolated from animals after 7 days of parenteral nutrition.

RESULTS

No differences in liver function test values of PN and PNC groups were detected compared with group fed rat food. However, lidocaine metabolism was found to be significantly reduced in both the PN and PNC groups. Significant reductions were observed in the hepatic extraction ratio (23% and 15%) and in intrinsic clearance (61% and 53%) in PN and PNC animals, respectively (p < .05). Material balance at steady state showed that recovery of lidocaine was threefold higher in the PN group and twofold higher in the PNC group than the rat food group (p < .05). Metabolite-to-drug ratios were determined for each lidocaine metabolite and this revealed significant reductions in N-dealkylation (64% and 57%) and aryl methyl hydroxylation (92% and 86%) in PN and PNC animals, respectively (p < .05).

CONCLUSIONS

Histologic findings suggest that cholestyramine feeding prevented liver dysfunction, possibly through interruption of secondary bile salt reabsorption. However, lidocaine metabolism was still impaired after cholestyramine ingestion; the impairment mechanism remains unknown.

Cholesterol starvation induces differentiation of the intestinal parasite Giardia lamblia

Giardia lamblia, like most human intestinal parasitic protozoa, sustains fundamental morphological and biochemical changes to survive outside the small intestine of its mammalian host by differentiating into an infective cyst. However, the stimulus that triggers this differentiation remains totally undefined. In this work, we demonstrate the induction of cyst formation in vitro when trophozoites are starved for cholesterol. Expression of cyst wall proteins was detected within encystation-specific secretory vesicles 90 min after the cells were placed in lipoprotein-deficient TYI-S-33 medium. Four cloned lines derived from two independent Giardia isolates were tested, and all formed cysts similarly. Addition of cholesterol, low density or very low density lipoproteins to the lipoprotein-deficient culture medium, inhibited the expression of cyst wall proteins, the generation of encystation-specific vesicles, and cyst wall biogenesis. In contrast, high density lipoproteins, phospholipids, bile salts, or fatty acids had little or no effect. These results indicate that cholesterol starvation is necessary and sufficient for the stimulation of Giardia encystation in vitro and, likely, in the intestine of mammalian hosts.