Adaptive regulation of the ileal apical sodium dependent bile acid transporter (ASBT) in patients with obstructive cholestasis

Beyond nanoparticle-based oral drug delivery: transporter-mediated absorption and disease targeting

Various strategies at the microscale/nanoscale have been developed to improve oral absorption of therapeutics. Among them, gastrointestinal (GI)-transporter/receptor-mediated nanosized drug delivery systems (NDDSs) have drawn attention due to their many benefits, such as improved water solubility, improved chemical/physical stability, improved oral absorption, and improved targetability of their payloads. Their therapeutic potential in disease animal models (e.g., solid tumors, virus-infected lungs, metastasis, diabetes, and so on) has been investigated, and could be expanded to disease targeting after systemic/lymphatic circulation, although the detailed paths and mechanisms of endocytosis, endosomal escape, intracellular trafficking, and exocytosis through the epithelial cell lining in the GI tract are still unclear. Thus, this review summarizes and discusses potential GI transporters/receptors, their absorption and distribution, in vivo studies, and potential sequential targeting (e.g., oral absorption and disease targeting in organs/tissues).

Exploring Advanced Therapies for Primary Biliary Cholangitis: Insights from the Gut Microbiota-Bile Acid-Immunity Network

Primary biliary cholangitis (PBC) is a cholestatic liver disease characterized by immune-mediated injury to small bile ducts. Although PBC is an autoimmune disease, the effectiveness of conventional immunosuppressive therapy is disappointing. Nearly 40% of PBC patients do not respond to the first-line drug UDCA. Without appropriate intervention, PBC patients eventually progress to liver cirrhosis and even death. There is an urgent need to develop new therapies. The gut-liver axis emphasizes the interconnection between the gut and the liver, and evidence is increasing that gut microbiota and bile acids play an important role in the pathogenesis of cholestatic diseases. Dysbiosis of gut microbiota, imbalance of bile acids, and immune-mediated bile duct injury constitute the triad of pathophysiology in PBC. Autoimmune cholangitis has the potential to be improved through immune system modulation. Considering the failure of conventional immunotherapies and the involvement of gut microbiota and bile acids in the pathogenesis, targeting immune factors associated with them, such as bile acid receptors, microbial-derived molecules, and related specific immune cells, may offer breakthroughs. Understanding the gut microbiota-bile acid network and related immune dysfunctions in PBC provides a new perspective on therapeutic strategies. Therefore, we summarize the latest advances in research of gut microbiota and bile acids in PBC and, for the first time, explore the possibility of related immune factors as novel immunotherapy targets. This article discusses potential therapeutic approaches focusing on regulating gut microbiota, maintaining bile acid homeostasis, their interactions, and related immune factors.

Paclitaxel chemotherapy disrupts microbiota-enterohepatic bile acid metabolism in mice

Balanced interactions between the enteric microbiota and enterohepatic organs are essential to bile acid homeostasis, and thus normal gastrointestinal function. Disruption of these interactions by cancer treatment instigates bile acid malabsorption, leading to treatment delays, malnutrition, and decreased quality of life. However, the nature of chemotherapy-induced bile acid malabsorption remains poorly characterized with limited treatment options. Therefore, this study sought to characterize changes in hepatic, enteric, and microbial bile acid metabolism in a mouse model of chemotherapy-induced toxicity. Consistent with clinical bile acid malabsorption, chemotherapy increased fecal excretion of primary bile acids and water, while diminishing microbiome diversity, secondary bile acid formation, and small intestinal bile acid signaling. We identified new contributors to pathology of bile acid malabsorption in the forms of lipopolysaccharide-induced cholestasis and colonic crypt hyperplasia from reduced secondary bile acid signaling. Chemotherapy reduced markers of hepatic bile flow and bile acid synthesis, elevated markers of fibrosis and endotoxemia, and altered transcription of genes at all stages of bile acid metabolism. Primary hepatocytes exposed to lipopolysaccharide (but not chemotherapy) replicated chemotherapy-induced transcriptional differences, while gut microbial transplant into germ-free mice replicated very few differences. In the colon, chemotherapy-altered bile acid profiles (particularly higher tauromuricholic acid and lower hyodeoxycholic acid) coincided with crypt hyperplasia. Exposing primary colonoids to hyodeoxycholic acid reduced proliferation, while gut microbiota transplant enhanced proliferation. Together, these investigations reveal complex involvement of the entire microbiota-enterohepatic axis in chemotherapy-induced bile acid malabsorption. Interventions to reduce hepatic lipopolysaccharide exposure and enhance microbial bile acid metabolism represent promising co-therapies to cancer treatment.

The regulation of tissue-specific farnesoid X receptor on genes and diseases involved in bile acid homeostasis

Bile acids (BAs) facilitate the absorption of dietary lipids and vitamins and have also been identified as signaling molecules involved in regulating their own metabolism, glucose and lipid metabolism, as well as immunity. Disturbances in BA homeostasis are associated with various enterohepatic and metabolic diseases, such as cholestasis, nonalcoholic steatohepatitis, inflammatory bowel disease, and obesity. As a key regulator, the nuclear orphan receptor farnesoid X receptor (FXR, NR1H4) precisely regulates BA homeostasis by transcriptional regulation of genes involved in BA synthesis, metabolism, and enterohepatic circulation. FXR is widely regarded as the most potential therapeutic target. Obeticholic acid is the only FXR agonist approved to treat patients with primary biliary cholangitis, but its non-specific activation of systemic FXR also causes high-frequency side effects. In recent years, developing tissue-specific FXR-targeting drugs has become a research highlight. This article provides a comprehensive overview of the role of tissue-specific intestine/liver FXR in regulating genes involved in BA homeostasis and briefly discusses tissue-specific FXR as a therapeutic target for treating diseases. These findings provide the basis for the development of tissue-specific FXR modulators for the treatment of enterohepatic and metabolic diseases associated with BA dysfunction.

The microbiota and the gut-liver axis in primary sclerosing cholangitis

Primary sclerosing cholangitis (PSC) offers unique opportunities to explore the gut-liver axis owing to the close association between liver disease and colonic inflammation. It is well established that the gut microbiota in people with PSC differs from that of healthy individuals, but details of the microbial factors that demarcate PSC from inflammatory bowel disease (IBD) without PSC are poorly understood. In this Review, we aim to provide an overview of the latest literature on the gut microbiome in PSC and PSC with IBD, critically examining hypotheses on how microorganisms could contribute to the pathogenesis of PSC. A particular emphasis will be put on pathogenic features of the gut microbiota that might explain the occurrence of bile duct inflammation and liver disease in the context of IBD, and we postulate the potential existence of a specific yet unknown factor related to the gut-liver axis as causative in PSC. Available data are scrutinized in the perspective of therapeutic approaches related to the gut-liver axis.

The Appendix Orchestrates T-Cell Mediated Immunosurveillance in Colitis-Associated Cancer

BACKGROUND & AIMS

Although appendectomy may reduce colorectal inflammation in patients with ulcerative colitis (UC), this surgical procedure has been suggested to be associated with an increased risk of colitis-associated cancer (CAC). Our aim was to explore the mechanism underlying the appendectomy-associated increased risk of CAC.

METHODS

Five-week-old male BALB/c mice underwent appendectomy, appendicitis induction, or sham laparotomy. They were then exposed to azoxymethane/dextran sodium sulfate (AOM/DSS) to induce CAC. Mice were killed 12 weeks later, and colons were taken for pathological analysis and immunohistochemistry (CD3 and CD8 staining). Human colonic tumors from 21 patients with UC who underwent surgical resection for CAC were immunophenotyped and stratified according to appendectomy status.

RESULTS

Whereas appendectomy significantly reduced colitis severity and increased CAC number, appendicitis induction without appendectomy led to opposite results. Intratumor CD3+ and CD8+ T-cell densities were lower after appendectomy and higher after appendicitis induction compared with the sham laparotomy group. Blocking lymphocyte trafficking to the colon with the anti-α4β7 integrin antibody or a sphingosine-1-phosphate receptor agonist suppressed the inducing effect of the appendectomy on tumors' number and on CD3+/CD8+ intratumoral density. CD8+ or CD3+ T cells isolated from inflammatory neo-appendix and intravenously injected into AOM/DSS-treated recipient mice increased CD3+/CD8+ T-cell tumor infiltration and decreased tumor number. In UC patients with a history of appendectomy, intratumor CD3+ and CD8+ T-cell densities were decreased compared with UC patients without history of appendectomy.

CONCLUSIONS

In UC, appendectomy could suppress a major site of T-cell priming, resulting in a less efficient CAC immunosurveillance.

Artificial intelligence-rationalized balanced PPARα/γ dual agonism resets dysregulated macrophage processes in inflammatory bowel disease

A computational platform, Boolean network explorer (BoNE), has recently been developed to infuse AI-enhanced precision into drug discovery; it enables invariant Boolean Implication Networks of disease maps for prioritizing high-value targets. Here we used BoNE to query an Inflammatory Bowel Disease (IBD)-map and prioritize a therapeutic strategy that involves dual agonism of two nuclear receptors, PPARα/γ. Balanced agonism of PPARα/γ was predicted to modulate macrophage processes, ameliorate colitis, 'reset' the gene expression network from disease to health. Predictions were validated using a balanced and potent PPARα/γ-dual-agonist (PAR5359) in Citrobacter rodentium- and DSS-induced murine colitis models. Using inhibitors and agonists, we show that balanced-dual agonism promotes bacterial clearance efficiently than individual agonists, both in vivo and in vitro. PPARα is required and sufficient to induce the pro-inflammatory cytokines and cellular ROS, which are essential for bacterial clearance and immunity, whereas PPARγ-agonism blunts these responses, delays microbial clearance; balanced dual agonism achieved controlled inflammation while protecting the gut barrier and 'reversal' of the transcriptomic network. Furthermore, dual agonism reversed the defective bacterial clearance observed in PBMCs derived from IBD patients. These findings not only deliver a macrophage modulator for use as barrier-protective therapy in IBD, but also highlight the potential of BoNE to rationalize combination therapy.

Placental Expression of Bile Acid Transporters in Intrahepatic Cholestasis of Pregnancy

Intrahepatic cholestasis of pregnancy (ICP) is a pregnancy-related condition characterized by increased maternal circulating bile acids (BAs) having adverse fetal effects. We investigated whether the human placenta expresses specific regulation patterns to prevent fetal exposition to harmful amounts of BAs during ICP. Using real-time quantitative PCR, we screened placentae from healthy pregnancies (n = 12) and corresponding trophoblast cells (n = 3) for the expression of 21 solute carriers and ATP-binding cassette transporter proteins, all acknowledged as BA- and/or cholestasis-related genes. The placental gene expression pattern was compared between healthy women and ICP patients (n = 12 each). Placental SLCO3A1 (OATP3A1) gene expression was significantly altered in ICP compared with controls. The other 20 genes, including SLC10A2 (ASBT) and EPHX1 (EPOX, mEH) reported for the first time in trophoblasts, were comparably abundant in healthy and ICP placentae. ABCG5 was undetectable in all placentae. Placental SLC10A2 (ASBT), SLCO4A1 (OATP4A1), and ABCC2 mRNA levels were positively correlated with BA concentrations in ICP. Placental SLC10A2 (ASBT) mRNA was also correlated with maternal body mass index. We conclude that at the transcriptional level only a limited response of BA transport systems is found under ICP conditions. However, the extent of the transcriptional response may also depend on the severity of the ICP condition and the magnitude by which the maternal BA levels are increased.

Alterations in Bile Acid Metabolism Associated With Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic relapsing inflammatory disorder closely related to gut dysbiosis, which is associated with alterations in an important bacterial metabolite, bile acids (BAs). Although certain findings pertinent to BA changes in IBD vary among studies owing to the differences in sample type, quantitated BA species, study methodology, and patient characteristics, a specific trend concerning variations of BAs in IBD has been identified. In elaborating on this observation, it was noted that primary BAs and conjugated BAs are augmented in fecal samples but there is a reduction in secondary BAs in fecal samples. It is not entirely clear why patients with IBD manifest these changes and what role these changes play in the onset and development of IBD. Previous studies have shown that IBD-associated BA changes may be caused by alterations in BA absorption, synthesis, and bacterial modification. The complex relationship between bacteria and BAs may provide additional and deeper insight into host-gut microbiota interactions in the pathogenesis of IBD. The characteristic BA changes may generate profound effects in patients with IBD by shaping the gut microbiota community, affecting inflammatory processes, causing BA malabsorption associated with diarrhea, and even leading to intestinal dysplasia and cancer. Thus, therapeutic strategies correcting the alterations in the composition of BAs, including the elimination of excess BAs and the supplementation of deficient BAs, may prove promising in IBD.

Small intestinal taurochenodeoxycholic acid-FXR axis alters local nutrient-sensing glucoregulatory pathways in rats

Objective

The mechanism of nutrient sensing in the upper small intestine (USI) and ileum that regulates glucose homeostasis remains elusive. Short-term high-fat (HF) feeding increases taurochenodeoxycholic acid (TCDCA; an agonist of farnesoid X receptor (FXR)) in the USI and ileum of rats, and the increase of TCDCA is prevented by transplantation of microbiota obtained from the USI of healthy donors into the USI of HF rats. However, whether changes of TCDCA-FXR axis in the USI and ileum alter nutrient sensing remains unknown.

Methods

Intravenous glucose tolerance test was performed in rats that received USI or ileal infusion of nutrients (i.e., oleic acids or glucose) via catheters placed toward the lumen of USI and/or ileum, while mechanistic gain- and loss-of-function studies targeting the TCDCA-FXR axis or bile salt hydrolase activity in USI and ileum were performed.

Results

USI or ileum infusion of nutrients increased glucose tolerance in healthy but not HF rats. Transplantation of healthy microbiome obtained from USI into the USI of HF rats restored nutrient sensing and inhibited FXR via a reduction of TCDCA in the USI and ileum. Further, inhibition of USI and ileal FXR enhanced nutrient sensing in HF rats, while inhibiting USI (but not ileal) bile salt hydrolase of HF rats transplanted with healthy microbiome activated FXR and disrupted nutrient sensing in the USI and ileum.

Conclusions

We reveal a TCDCA-FXR axis in both the USI and ileum that is necessary for the upper small intestinal microbiome to govern local nutrient-sensing glucoregulatory pathways in rats.

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Upper small intestinal infusion of oleic acid or glucose increases glucose tolerance in healthy but not HF-fed rats.

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Ileal infusion of oleic acid or glucose increases glucose tolerance in healthy but not HF-fed rats.

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Upper small intestinal healthy microbiome transplant enhances nutrient sensing and inhibits FXR via reduced TCDCA levels.

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Inhibition of FXR in the upper small intestine or ileum enhances oleic acid sensing to increase glucose tolerance.

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Inhibition of upper small intestinal bile salt hydrolase negates oleic acid sensing and activates FXR in the small intestine.

IL-13 as Target to Reduce Cholestasis and Dysbiosis in Abcb4 Knockout Mice

The Th2 cytokine IL-13 is involved in biliary epithelial injury and liver fibrosis in patients as well as in animal models. The aim of this study was to investigate IL-13 as a therapeutic target during short term and chronic intrahepatic cholestasis in an Abcb4-knockout mouse model (Abcb4^-/-). Lack of IL-13 protected Abcb4^-/- mice transiently from cholestasis. This decrease in serum bile acids was accompanied by an enhanced excretion of bile acids and a normalization of fecal bile acid composition. In Abcb4^-/-/IL-13^-/- double knockout mice, bacterial translocation to the liver was significantly reduced and the intestinal microbiome resembled the commensal composition in wild type animals. In addition, 52-week-old Abcb4^-/-IL-13^-/- mice showed significantly reduced hepatic fibrosis. Abcb4^-/- mice devoid of IL-13 transiently improved cholestasis and converted the composition of the gut microbiota towards healthy conditions. This highlights IL-13 as a potential therapeutic target in biliary diseases.

IBD and Bile Acid Absorption: Focus on Pre-clinical and Clinical Observations

Inflammatory bowel disease (IBD) causes chronic inflammation affecting the GI tract. It is classified as consisting of Crohn’s Disease (CD) and Ulcerative Colitis (UC). Bile Acid absorption is altered in both pre-clinical models of Inflammatory Bowel Disease (IB) and in human IBD. The bile acid transporter apical sodium dependent bile acid transporter (ASBT) showed decreased expression in rats with TNBS colitis. Decreased ASBT expression has also been described in murine, canine and rabbit models of intestinal inflammation. Human IBD studies have shown that an inflamed ileum can interrupt enterohepatic recirculation of bile acid, which could be due to inflammatory cytokine induced repression of the ASBT promoter. There are different hypotheses as to why ASBT is downregulated during CD. In addition, one study has demonstrated the beneficial effect of a glucocorticoid on ASBT expression, when treating IBD. Our aim in this paper was to systematically review various aspects of bile acid malabsorption in animal models of intestinal inflammation, as well as in IBD.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Dysregulated bile acid signaling contributes to the neurological impairment in murine models of acute and chronic liver failure

BACKGROUND

Hepatic encephalopathy (HE), a severe neuropsychiatric complication, is associated with increased blood levels of ammonia and bile acids (BAs). We sought to determine (1) whether abnormally increased blood BAs in liver cirrhotic patients with HE is caused by elevation of apical sodium-dependent BA transporter (ASBT)-mediated BA reabsorption; and (2) whether increased BA reabsorption would exacerbate ammonia-induced brain injuries.

METHODS

We quantitatively measured blood BA and ammonia levels in liver cirrhosis patients with or without HE and healthy controls. We characterized ASBT expression, BA profiles, and ammonia concentrations in a chronic liver disease (CLD) mouse model induced by streptozotocin-high fat diet (STZ-HFD) and an azoxymethane (AOM) - induced acute liver failure (ALF) mouse model. These two mouse models were treated with SC-435 (ASBT inhibitor) and budesonide (ASBT activator), respectively.

FINDINGS

Blood concentrations of ammonia and conjugated BAs were substantially increased in cirrhotic patients with HE (n = 75) compared to cirrhotic patients without HE (n = 126). Pharmacological inhibition of the enterohepatic BA circulation using a luminal- restricted ASBT inhibitor, SC-435, in mice with AOM-induced ALF and STZ-HFD -induced CLD effectively reduced BA and ammonia concentrations in the blood and brain, and alleviated liver and brain damages. Budesonide treatment induced liver and brain damages in normal mice, and exacerbated these damages in AOM-treated mice.

INTERPRETATION

ASBT mediated BA reabsorption increases intestinal luminal pH and facilitates conversion of intestinal ammonium to ammonia, leading to abnormally high levels of neurotoxic ammonia and cytotoxic BAs in the blood and brain. Inhibition of intestinal ASBT with SC-435 can effectively remove neurotoxic BAs and ammonia from the bloodstream and thus, mitigate liver and brain injuries resulting from liver failure.

Comparison of intestinal expression of the apical sodium-dependent bile acid transporter between dogs with and without chronic inflammatory enteropathy

BACKGROUND

Intestinal absorption of bile acids is mediated by the apical sodium-dependent bile acid transporter (ASBT). Fecal bile acid dysmetabolism has been reported in dogs with chronic inflammatory enteropathy (CIE).

OBJECTIVE

Characterization of ASBT distribution along the intestinal tract of control dogs and comparison to dogs with CIE.

ANIMALS

Twenty-four dogs with CIE and 11 control dogs.

METHODS

The ASBT mRNA and protein expression were assessed using RNA in situ hybridization and immunohistochemistry, respectively. The concentrations of fecal bile acids were measured by gas chromatography-mass spectrometry. The fecal microbiota dysbiosis index was assessed with a quantitative polymerase chain reaction panel.

RESULTS

In control dogs, ASBT mRNA expression was observed in enterocytes in all analyzed intestinal segments, with highest expression in the ileum. The ASBT protein expression was restricted to enterocytes in the ileum, cecum, and colon. Dogs with CIE had significantly decreased expression of ASBT protein in the ileum (P = .001), which was negatively correlated with histopathological score (ρ = -0.40; Pcorr = .049). Additionally, dogs with CIE had a significantly increased percentage of primary bile acids in feces compared to controls (P = .04). The fecal dysbiosis index was significantly higher in dogs with CIE than in control dogs (P = .01).

CONCLUSIONS AND CLINICAL IMPORTANCE

These findings indicate that ileal protein expression of ASBT is downregulated in dogs with CIE. This change may be linked to the inflammatory process, intestinal dysbiosis, and fecal bile acid dysmetabolism observed in these patients.

Inflammatory Bowel Disease and Primary Sclerosing Cholangitis: A Review of the Phenotype and Associated Specific Features

Primary sclerosing cholangitis (PSC) is a chronic, progressive cholestatic disease that is associated with inflammatory bowel disease (IBD) in approximately 70% of cases. Although the pathogenesis is still unknown for both diseases, there is increasing evidence to indicate that they share a common underlying predisposition. Herein, we review the epidemiology, diagnosis, disease pathogenesis, and specific clinical features of the PSC-IBD phenotype. Patients with PSC-IBD have a distinct IBD phenotype with an increased incidence of pancolitis, backwash ileitis, and rectal sparing. Despite often having extensive colonic involvement, these patients present with mild intestinal symptoms or are even asymptomatic, which can delay the diagnosis of IBD. Although the IBD phenotype has been well characterized in PSC patients, the natural history and disease behavior of PSC in PSC-IBD patients is less well defined. There is conflicting evidence regarding the course of IBD in PSC-IBD patients who receive liver transplantation and their risk of recurrent PSC. IBD may also be associated with an increased risk of cholangiocarcinoma in PSC patients. Overall, the PSC-IBD population has an increased risk of developing colorectal neoplasia compared to the conventional IBD population. Lifelong annual surveillance colonoscopy is currently recommended.

Na+ -taurocholate cotransporting polypeptide inhibition has hepatoprotective effects in cholestasis in mice

Accumulation of bile salts (BSs) during cholestasis leads to hepatic and biliary injury, driving inflammatory and fibrotic processes. The Na⁺ -Taurocholate Cotransporting Polypeptide (NTCP) is the major hepatic uptake transporter of BSs, and can be specifically inhibited by myrcludex B. We hypothesized that inhibition of NTCP dampens cholestatic liver injury. Acute cholestasis was induced in mice by a 3.5-diethoxycarbonyl-1.4-dihydrocollidine (DDC) diet or by bile duct ligation (BDL). Chronic cholestasis was investigated in Atp8b1-G308V and Abcb4/Mdr2 deficient mice. Mice were injected daily with myrcludex B or vehicle. Myrcludex B reduced plasma alkaline phosphatase (ALP) levels in DDC-fed, Atp8b1-G308V and BDL mice by 39%, 27% and 48% respectively. Expression of genes involved in fibrosis, proliferation and inflammation was reduced by myrcludex B treatment in DDC-fed and Atp8b1-G308V mice. NTCP-inhibition increased plasma BS levels from 604±277 to 1746±719 μm in DDC-fed mice, 432±280 to 762±288 μm in Atp8b1-G308V mice and from 522±130 to 3625±378 μm in BDL mice. NTCP-inhibition strongly aggravated weight loss in BDL mice, but not in other cholestatic models studied. NTCP-inhibition reduced biliary BS output in DDC-fed and Atp8b1-G308V mice by ∼50% while phospholipid (PL) output was maintained, resulting in a higher PL/BS ratio. Conversely, liver injury in Abcb4 deficient mice, lacking biliary phospholipid output, was aggravated after myrcludex B treatment. Conclusion: NTCP-inhibition by myrcludex B has hepatoprotective effects, by reducing BS load in hepatocytes and increasing the biliary PL/BS ratio. High micromolar plasma BS levels after NTCP-inhibition were well tolerated. NTCP-inhibition may be beneficial in selected forms of cholestasis. (Hepatology 2018).

An important intestinal transporter that regulates the enterohepatic circulation of bile acids and cholesterol homeostasis: The apical sodium-dependent bile acid transporter (SLC10A2/ASBT)

The enterohepatic circulation of bile acids (BAs) is governed by specific transporters expressed in the liver and the intestine and plays a critical role in the digestion of fats and oils. During this process, the majority of the BAs secreted from the liver is reabsorbed in intestinal epithelial cells via the apical sodium-dependent bile acid transporter (ASBT/SLC10A2) and then transported into the portal vein. Previous studies revealed that regulation of the ASBT involves BAs and cholesterol. In addition, abnormal ASBT expression and function might lead to some diseases associated with disorders in the enterohepatic circulation of BAs and cholesterol homeostasis, such as diarrhoea and gallstones. However, decreasing cholesterol or BAs by partly inhibiting ASBT-mediated transport might be used for treatments of hypercholesterolemia, cholestasis and diabetes. This review mainly discusses the regulation of the ASBT by BAs and cholesterol and its relevance to diseases and treatment.

The gut microbiota, bile acids and their correlation in primary sclerosing cholangitis associated with inflammatory bowel disease

Background

Patients with primary sclerosing cholangitis associated with inflammatory bowel disease (PSC-IBD) have a very high risk of developing colorectal neoplasia. Alterations in the gut microbiota and/or gut bile acids could account for the increase in this risk. However, no studies have yet investigated the net result of cholestasis and a potentially altered bile acid pool interacting with a dysbiotic gut flora in the inflamed colon of PSC-IBD.

Aim

The aim of this study was to compare the gut microbiota and stool bile acid profiles, as well as and their correlation in patients with PSC-IBD and inflammatory bowel disease alone.

Methods

Thirty patients with extensive colitis (15 with concomitant primary sclerosing cholangitis) were prospectively recruited and fresh stool samples were collected. The microbiota composition in stool was profiled using bacterial 16S rRNA sequencing. Stool bile acids were assessed by high-performance liquid chromatography tandem mass spectrometry.

Results

The total stool bile acid pool was significantly reduced in PSC-IBD. Although no major differences were observed in the individual bile acid species in stool, their overall combination allowed a good separation between PSC-IBD and inflammatory bowel disease. Compared with inflammatory bowel disease alone, PSC-IBD patients demonstrated a different gut microbiota composition with enrichment in Ruminococcus and Fusobacterium genus compared with inflammatory bowel disease. At the operational taxonomic unit level major shifts were observed within the Firmicutes (73%) and Bacteroidetes phyla (17%). Specific microbiota-bile acid correlations were observed in PSC-IBD, where 12% of the operational taxonomic units strongly correlated with stool bile acids, compared with only 0.4% in non-PSC-IBD.

Conclusions

Patients with PSC-IBD had distinct microbiota and microbiota-stool bile acid correlations as compared with inflammatory bowel disease. Whether these changes are associated with, or may predispose to, an increased risk of colorectal neoplasia needs to be further clarified.

The features of mucosa-associated microbiota in primary sclerosing cholangitis

BACKGROUND

Little is known about the role of the microbiome in primary sclerosing cholangitis.

AIM

To explore the mucosa-associated microbiota in primary sclerosing cholangitis (PSC) patients across different locations in the gut, and to compare it with inflammatory bowel disease (IBD)-only patients and healthy controls.

METHODS

Biopsies from the terminal ileum, right colon, and left colon were collected from patients and healthy controls undergoing colonoscopy. Microbiota profiling using bacterial 16S rRNA sequencing was performed on all biopsies.

RESULTS

Forty-four patients were recruited: 20 with PSC (19 with PSC-IBD and one with PSC-only), 15 with IBD-only and nine healthy controls. The overall microbiome profile was similar throughout different locations in the gut. No differences in the global microbiome profile were found. However, we observed significant PSC-associated enrichment in Barnesiellaceae at the family level, and in Blautia and an unidentified Barnesiellaceae at the genus level. At the operational taxa unit level, most shifts in PSC were observed in Clostridiales and Bacteroidales orders, with approximately 86% of shifts occurring within the former order.

CONCLUSIONS

The overall microbiota profile was similar across multiple locations in the gut from the same individual regardless of disease status. In this study, the mucosa associated-microbiota of patients with primary sclerosing cholangitis was characterised by enrichment of Blautia and Barnesiellaceae and by major shifts in operational taxa units within Clostridiales order.

Increased risk of colorectal neoplasia in patients with primary sclerosing cholangitis and inflammatory bowel disease: a meta-analysis of 16 observational studies

Ulcerative colitis (UC) patients with concomitant primary sclerosing cholangitis (PSC) carry an increased risk of colorectal neoplasia (dysplasia and cancer), whereas the association between PSC and the development of colorectal neoplasia in Crohn's disease (CD) is controversial. A meta-analysis was carried out to compare the risk of this neoplasia in patients with inflammatory bowel disease (IBD) with and without PSC. A systematic research of MEDLINE, EMBASE and the Cochrane Central Register of Controlled Trials was performed to identify studies that compared the risk of colorectal neoplasia (dysplasia and cancer) in patients with IBD with and without PSC. Quality assessment was performed using the Newcastle-Ottawa Scale. Pooled odds ratio (OR) was calculated using the random-effects model by STATA 12.0. A total of 16 studies (four cohort studies, 12 case-control studies; nine prospective studies and seven retrospective studies) were selected for further study. These studies included 13 379 IBD patients, of whom 1022 also had PSC. Patients with IBD and PSC were at an increased risk of colorectal dysplasia and cancer compared with patients with IBD alone [OR 3.24; 95% confidence interval (CI): 2.14-4.90]. This increased risk was present even when the risk of colorectal cancer alone was analysed (OR 3.41; 95% CI: 2.13-5.48). Data only from patients with UC showed that PSC was associated with an increased risk for the development of colorectal neoplasia and cancer in patients with UC (OR 2.98; 95% CI: 1.54-5.76) (OR 3.01; 95% CI: 1.44-6.29), but there were high heterogeneity among studies (I=76.9 and 62.8%, respectively). Heterogeneity of the studies was affected by the study design (prospective or retrospective). The OR of colorectal neoplasia was 2.32 (95% CI: 0.70-7.70, P=0.133) and that of cancer was 2.91 (95% CI: 0.84-10.16, P=0.388) for patients with CD and concurrent PSC. Patients with IBD and PSC have a markedly higher risk for the development of colorectal neoplasia than patients with IBD, but not PSC. Stratification by IBD type show that the presence of PSC is associated with an increased risk for the development of colorectal neoplasia in patients with UC; however, there is a nonsignificant association in CD patients. When the risk of colorectal cancer alone is analysed, the conclusion does not change.

Characterization of primary structure and tissue expression profile of the chicken apical sodium-dependent bile acid transporter mRNA

The ileal apical sodium-dependent bile acid cotransporter (ASBT) plays an essential role in the absorption of bile acids from intestinal lumina. ASBT cDNA has been cloned from mammalian and fish species, and the primary structure of the protein and expression properties of the mRNA have been characterized. In this study, we identified chicken ASBT mRNA by cDNA cloning. Chicken ASBT cDNA consisted of 91 bp of the 5'-untranslated region, 1,083 bp of the coding region, and 1,896 bp of the 3'-untranslated region. The cDNA encoded a protein of 360 amino acids showing significant sequence identity with mammalian and fish ASBT. The amino acid residues known to participate in the functions of mammalian ASBT were conserved in chicken ASBT. Real-time polymerase chain reaction analysis revealed that chicken ASBT mRNA was expressed at markedly higher levels in the ileum and proximal colon/rectum, relatively lower levels in the kidney, and very low levels in the jejunum and cecum. Expression levels in the ileum markedly increased after hatching, reached the highest levels on day 7 posthatching, and then decreased to adult levels. A similar expression pattern was observed in the proximal colon/rectum except for the significant decrease from day 7 posthatching to day 21 posthatching. These results suggest that chicken ASBT functions as a bile acid transporter in the ileum and proximal colon/rectum, particularly during the early posthatching period.

Bile acid signaling through farnesoid X and TGR5 receptors in hepatobiliary and intestinal diseases

BACKGROUND

The well-known functions of bile acids (BAs) are the emulsification and absorption of lipophilic xenobiotics. However, the emerging evidences in the past decade showed that BAs act as signaling molecules that not only autoregulate their own metabolism and enterohepatic recirculation, but also as important regulators of integrative metabolism by activating nuclear and membrane-bound G protein-coupled receptors. The present review was to get insight into the role of maintenance of BA homeostasis and BA signaling pathways in development and management of hepatobiliary and intestinal diseases.

DATA SOURCES

Detailed and comprehensive search of PubMed and Scopus databases was carried out for original and review articles.

RESULTS

Disturbances in BA homeostasis contribute to the development of several hepatobiliary and intestinal disorders, such as non-alcoholic fatty liver disease, liver cirrhosis, cholesterol gallstone disease, intestinal diseases and both hepatocellular and colorectal carcinoma.

CONCLUSION

Further efforts made in order to advance the understanding of sophisticated BA signaling network may be promising in developing novel therapeutic strategies related not only to hepatobiliary and gastrointestinal but also systemic diseases.

New therapeutic avenues in ulcerative colitis: thinking out of the box

Ulcerative colitis (UC) is a chronic idiopathic inflammatory disease of the gastrointestinal tract that affects the mucosal lining of the colon. Recent epidemiological data show that its incidence and prevalence are increasing in many parts of the world, in parallel with altered lifestyles, improved access to health, improved sanitation and industrialisation rates. Current therapeutic strategies for treating UC have only been moderately successful. Despite major recent advances in inflammatory bowel disease therapeutic resources, a considerable proportion of patients are still refractory to conventional treatment. Less than half of all patients achieve long-term remission, many require colectomy, and the disease still has a major impact on patients' lives. Moreover, recent data point to slightly raised mortality. While these outcomes could be partly improved by optimising current therapeutic strategies, they clearly highlight the need to develop new therapies. Currently, a number of promising and innovative therapeutic approaches are being explored, some of which will hopefully survive to reach the clinic. Until such a time arrives, it is important that a better understanding of the clinical particularities of the disease, an improved knowledge of the host-microbiome negative interactions and of the environmental factors beyond disease development is achieved to obtain the final and desired outcome: to provide better treatment and quality of life for patients with this disabling disease.

Alterations in bile acid synthesis in carriers of hepatocyte nuclear factor 1α mutations

OBJECTIVES

Heterozygous mutations in hepatocyte nuclear factor 1α (HNF1α) cause maturity onset diabetes of the young 3 (MODY3), an autosomal dominant form of diabetes. Deficiency of HNF1α in mice results in diabetes, hypercholesterolaemia and increased bile acid (BA) and cholesterol synthesis. Little is known about alterations in lipid metabolism in patients with MODY3. The aim of this study was to investigate whether patients with MODY3 have altered cholesterol and BA synthesis and intestinal cholesterol absorption. A secondary aim was to investigate the effects of HNF1α mutations on the transcriptional regulation of BA metabolism.

METHODS

Plasma biomarkers of BA and cholesterol synthesis and intestinal cholesterol absorption were measured in patients with MODY3 (n = 19) and in matched healthy control subjects (n = 15). Cotransfection experiments were performed with several promoters involved in BA metabolism along with expression vectors carrying the mutations found in these patients.

RESULTS

Plasma analysis showed higher levels of BA synthesis in patients with MODY3. No differences were observed in cholesterol synthesis or intestinal cholesterol absorption. Cotransfection experiments showed that one of the mutations (P379A) increased the induction of the cholesterol 7α-hydroxylase promoter compared with HNF1α, without further differences in other studied promoters. By contrast, the other four mutations (L107I, T260M, P291fsinsC and R131Q) reduced the induction of the farnesoid X receptor (FXR) promoter, which was followed by reduced repression of the small heterodimer partner promoter. In addition, these mutations also reduced the induction of the apical sodium-dependent bile salt transporter promoter.

CONCLUSIONS

BA synthesis is increased in patients with MODY3 compared with control subjects. Mutations in HNF1α affect promoters involved in BA metabolism.

Alagille syndrome and other hereditary causes of cholestasis

Neonatal conjugated jaundice is a common presentation of hereditary liver diseases, which, although rare, are important to recognize early. Developments in molecular genetic techniques have enabled the identification of causative genes, which has improved diagnostic accuracy for patients and has led to a greater understanding of the molecular pathways involved in liver biology and pathogenesis of liver diseases. This review provides an update of the current understanding of clinical and molecular features of the inherited liver diseases that cause neonatal conjugated jaundice.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

Farnesoid X receptor expression is decreased in colonic mucosa of patients with primary sclerosing cholangitis and colitis-associated neoplasia

BACKGROUND

The expression and distribution of farnesoid X receptor (FXR) in colitis and colitis-associated neoplasia (CAN) is unknown. We investigated FXR expression in neoplastic and nonneoplastic tissue from ulcerative colitis (UC) patients, with or without primary sclerosing cholangitis (PSC), as well as the role of DNA methylation in FXR expression in colorectal cancer (CRC) cell lines.

METHODS

Samples from the right (RC) and left (LC) colon of patients with UC, with and without PSC, and with or without CAN, were stained by immunohistochemistry and scored semiquantitatively for nuclear FXR expression. FXR expression was analyzed by western blot and polymerase chain reaction (PCR) in nine different CRC cell lines before and after demethylation with 5-azacytidine.

RESULTS

In nondysplastic samples, FXR expression demonstrated a diminishing expression from proximal to distal colon (strong FXR expression: 39% RC samples vs. 14% LC samples; P = 0.007). With moderate-to-severe inflammation, FXR expression was almost always absent or weak in both UC and PSC-UC, regardless of location. With quiescent/mild inflammation, 56% of UC samples in the RC retained strong FXR expression versus 24% of PSC-UC samples (P= 0.017). FXR was absent in 72% of the neoplastic samples, with an inverse association with the grade of dysplasia. FXR expression was absent in all CRC cell lines, in some cases due to DNA methylation.

CONCLUSIONS

FXR expression is inversely correlated with neoplastic progression and severity of inflammation in UC. Patients with PSC-UC have diminished FXR expression in the proximal colon compared to UC patients. This finding could contribute to the higher risk of proximal neoplasia in PSC patients.

The molecular pathogenesis of cholestasis in sepsis

Sepsis-induced cholestasis is a complication of infection. Infections cause systemic and intrahepatic increase in proinflammatory cytokines which result in impaired bile flow ie. cholestasis. Several other mediators of impairment in bile flow have been identified under conditions of sepsis such as increased nitric oxide production and decreased aquaporin channels. The development of cholestasis may also further worsen inflammation. The molecular basis of normal bile flow and mechanisms of impairment in sepsis are discussed.

Recent advances in small bowel diseases: Part II

As is the case in all areas of gastroenterology and hepatology, in 2009 and 2010 there were many advances in our knowledge and understanding of small intestinal diseases. Over 1000 publications were reviewed, and the important advances in basic science as well as clinical applications were considered. In Part II we review six topics: absorption, short bowel syndrome, smooth muscle function and intestinal motility, tumors, diagnostic imaging, and cystic fibrosis.

Effect of ursodeoxycholic acid on bile acid profiles and intestinal detoxification machinery in primary biliary cirrhosis and health

BACKGROUND & AIMS

Ursodeoxycholic acid (UDCA) exerts anticholestatic, antifibrotic and antiproliferative effects in primary biliary cirrhosis (PBC) via mechanisms not yet fully understood. Its adequate biliary enrichment is considered mandatory for therapeutic efficacy. However, precise determination of biliary enrichment of UDCA is not possible in clinical practice. Therefore, we investigated (i) the relationship between biliary enrichment and plasma pharmacokinetics of UDCA, (ii) the effect of UDCA on plasma and biliary bile acid composition and conjugation patterns, and (iii) on the intestinal detoxification machinery in patients with PBC and healthy controls.

METHODS

In 11 PBC patients and 11 matched healthy subjects, cystic bile and duodenal tissue were collected before and after 3 weeks of administration of UDCA (15 mg/kg/day). Extensive pharmacokinetic profiling of bile acids was performed. The effect of UDCA on the intestinal detoxification machinery was studied by quantitative PCR and Western blotting.

RESULTS

The relative fraction of UDCA and its conjugates in plasma at trough level[x] correlated with their biliary enrichment[y] (r=0.73, p=0.0001, y=3.65+0.49x). Taurine conjugates of the major hydrophobic bile acid, chenodeoxycholic acid, were more prominent in bile of PBC patients than in that of healthy controls. Biliary bile acid conjugation patterns normalized after treatment with UDCA. UDCA induced duodenal expression of key export pumps, BCRP and P-glycoprotein.

CONCLUSIONS

Biliary and trough plasma enrichment of UDCA are closely correlated in PBC and health. Taurine conjugation may represent an adaptive mechanism in PBC against chenodeoxycholic acid-mediated bile duct damage. UDCA may stabilize small intestinal detoxification by upregulation of efflux pumps.

Intestinal transporters for endogenic and pharmaceutical organic anions: the challenges of deriving in-vitro kinetic parameters for the prediction of clinically relevant drug-drug interactions

OBJECTIVES

This review provides an overview of intestinal human transporters for organic anions and stresses the need for standardization of the various in-vitro methods presently employed in drug-drug interaction (DDI) investigations.

KEY FINDINGS

Current knowledge on the intestinal expression of the apical sodium-dependent bile acid transporter (ASBT), the breast cancer resistance protein (BCRP), the monocarboxylate transporters (MCT) 1, MCT3-5, the multidrug resistance associated proteins (MRP) 1-6, the organic anion transporting polypetides (OATP) 2B1, 1A2, 3A1 and 4A1, and the organic solute transporter α/β (OSTα/β) has been covered along with an overview of their substrates and inhibitors. Furthermore, the many challenges in predicting clinically relevant DDIs from in-vitro studies have been discussed with focus on intestinal transporters and the various methods for deducting in-vitro parameters for transporters (K(m) /K(i) /IC50, efflux ratio). The applicability of using a cut-off value (estimated based on the intestinal drug concentration divided by the K(i) or IC50) has also been considered.

SUMMARY

A re-evaluation of the current approaches for the prediction of DDIs is necessary when considering the involvement of other transporters than P-glycoprotein. Moreover, the interplay between various processes that a drug is subject to in-vivo such as translocation by several transporters and dissolution should be considered.

Functional characterization of genetic variants in the apical sodium-dependent bile acid transporter (ASBT; SLC10A2)

BACKGROUND AND AIM

The major transporter responsible for bile acid uptake from the intestinal lumen is the apical sodium-dependent bile acid transporter (ASBT, SLC10A2). Analysis of the SLC10A2 gene has identified a variety of sequence variants including coding region single nucleotide polymorphisms (SNPs) that may influence bile acid homeostasis/intestinal function. In this study, we systematically characterized the effect of coding SNPs on SLC10A2 protein expression and bile acid transport activity.

METHODS

Single nucleotide polymorphisms in SLC10A2 from genomic DNA of ethnically-defined healthy individuals were identified using a polymerase chain reaction (PCR)-based temperature gradient capillary electrophoresis (TGCE) system. A heterologous gene expression system was used to assess transport activity of SLC10A2 nonsynonymous variants and missense mutations. Total and cell surface protein expression of wild-type and variant ASBT was assessed by Western blot analysis and immunofluorescence confocal microscopy. Expression of ASBT mRNA and protein was also measured in human intestinal samples.

RESULTS

The studies revealed two nonsynonymous SNPs, 292G>A and 431G>A, with partially impaired in vitro taurocholate transport. A novel variant, 790A>G, was also shown to exhibit near complete loss of taurocholate transport, similar to the previously identified ASBT missense mutations. Examination of ASBT protein expression revealed no significant differences in expression or trafficking to the cell surface among variants versus wild-type ASBT. Analysis of ASBT mRNA and protein expression in human intestinal samples revealed modest intersubject variability.

CONCLUSIONS

Genome sequencing and in vitro studies reveal the presence of multiple functionally relevant variants in SLC10A2 that may influence bile acid homeostasis and physiology.

Review article: colorectal neoplasia in patients with primary sclerosing cholangitis and inflammatory bowel disease

BACKGROUND

Primary sclerosing cholangitis (PSC) is a chronic cholestatic liver disease strongly associated with inflammatory bowel disease (IBD). IBD patients diagnosed with PSC have an increased risk of colorectal dysplasia and cancer.

AIMS

To review the available evidence regarding colorectal neoplasia epidemiology, preventive strategies and outcomes in patients with PSC and IBD, and to advance some hypotheses regarding possible mechanisms involved in cancer pathogenesis in these patients.

METHODS

A PubMed search was conducted for the English language publications with predetermined search criteria. Reference lists from studies selected were manually searched to identify further relevant reports. Relevant manuscripts considering colorectal neoplasia in patients with PSC-IBD were selected.

RESULTS

Primary sclerosing cholangitis increases the risk of colorectal neoplasia in patients with ulcerative colitis; fewer data are available for Crohn's disease. PSC-IBD patients tend to be younger at diagnosis of IBD and at diagnosis of colorectal cancer. Colorectal cancer in PSC-IBD patients predominates in the right colon. The increased risk of neoplasia is maintained after liver transplant and proctocolectomy. The role of ursodeoxycholic acid as a chemopreventive agent is controversial. The mechanisms underlying increased risk of colorectal neoplasia in these patients remain unknown.

CONCLUSIONS

A more comprehensive understanding of the mechanisms involved in colorectal neoplasia development in PSC-IBD patients is needed. Until then, early cancer detection through enrolment in surveillance programmes is the only available strategy to decrease cancer risk.

Role of the intestinal bile acid transporters in bile acid and drug disposition

Membrane transporters expressed by the hepatocyte and enterocyte play critical roles in maintaining the enterohepatic circulation of bile acids, an effective recycling and conservation mechanism that largely restricts these potentially cytotoxic detergents to the intestinal and hepatobiliary compartments. In doing so, the hepatic and enterocyte transport systems ensure a continuous supply of bile acids to be used repeatedly during the digestion of multiple meals throughout the day. Absorption of bile acids from the intestinal lumen and export into the portal circulation is mediated by a series of transporters expressed on the enterocyte apical and basolateral membranes. The ileal apical sodium-dependent bile acid cotransporter (abbreviated ASBT; gene symbol, SLC10A2) is responsible for the initial uptake of bile acids across the enterocyte brush border membrane. The bile acids are then efficiently shuttled across the cell and exported across the basolateral membrane by the heteromeric Organic Solute Transporter, OSTα-OSTβ. This chapter briefly reviews the tissue expression, physiology, genetics, pathophysiology, and transport properties of the ASBT and OSTα-OSTβ. In addition, the chapter discusses the relationship between the intestinal bile acid transporters and drug metabolism, including development of ASBT inhibitors as novel hypocholesterolemic or hepatoprotective agents, prodrug targeting of the ASBT to increase oral bioavailability, and involvement of the intestinal bile acid transporters in drug absorption and drug-drug interactions.

Bile acids: short and long term effects in the intestine

Bile acids have secretory, motility and antimicrobial effects in the intestine. In patients with bile acid malabsorption the amount of primary bile acids in the colon is increased compared to healthy controls. Deoxycholic acid is affecting the intestinal smooth muscle activity. Chenodeoxycholic acid has the highest potency to affect intestinal secretion. Litocholic acid has little effect in the lumen of intestine compared to both deoxycholic acid and chenodeoxycholic acid. There is no firm evidence that clinically relevant concentrations of bile acids induce colon cancer. Alterations in bile acid metabolism may be involved in the pathophysiology of constipation.

Bile salts and cholestasis

Bile salts have a crucial role in hepatobiliary and intestinal homeostasis and digestion. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Bile salts are efficiently reabsorbed from the intestinal lumen to undergo enterohepatic circulation. In addition to their function as a surfactant involved in the absorption of dietary lipids and fat-soluble vitamins bile salts are potent signaling molecules in both the liver and intestine. Under physiological conditions the bile salt pool is tightly regulated, but the adaptive capacity may fall short under cholestatic conditions. Elevated serum and tissue levels of potentially toxic hydrophobic bile salts during cholestasis may cause mitochondrial damage, apoptosis or necrosis in susceptible cell types. Therapeutic nontoxic bile salts may restore impaired hepatobiliary secretion in cholestatic disorders. The hydrophilic bile salt ursodeoxycholate is today regarded as the effective standard treatment of primary biliary cirrhosis and intrahepatic cholestasis of pregnancy, and is implicated for use in various other cholestatic conditions. Novel therapeutic bile salts that are currently under evaluation may also prove valuable in the treatment of these diseases.

Bile acid transporters

In liver and intestine, transporters play a critical role in maintaining the enterohepatic circulation and bile acid homeostasis. Over the past two decades, there has been significant progress toward identifying the individual membrane transporters and unraveling their complex regulation. In the liver, bile acids are efficiently transported across the sinusoidal membrane by the Na(+) taurocholate cotransporting polypeptide with assistance by members of the organic anion transporting polypeptide family. The bile acids are then secreted in an ATP-dependent fashion across the canalicular membrane by the bile salt export pump. Following their movement with bile into the lumen of the small intestine, bile acids are almost quantitatively reclaimed in the ileum by the apical sodium-dependent bile acid transporter. The bile acids are shuttled across the enterocyte to the basolateral membrane and effluxed into the portal circulation by the recently indentified heteromeric organic solute transporter, OSTalpha-OSTbeta. In addition to the hepatocyte and enterocyte, subgroups of these bile acid transporters are expressed by the biliary, renal, and colonic epithelium where they contribute to maintaining bile acid homeostasis and play important cytoprotective roles. This article will review our current understanding of the physiological role and regulation of these important carriers.

Bile acid transporters

In liver and intestine, transporters play a critical role in maintaining the enterohepatic circulation and bile acid homeostasis. Over the past two decades, there has been significant progress toward identifying the individual membrane transporters and unraveling their complex regulation. In the liver, bile acids are efficiently transported across the sinusoidal membrane by the Na(+) taurocholate cotransporting polypeptide with assistance by members of the organic anion transporting polypeptide family. The bile acids are then secreted in an ATP-dependent fashion across the canalicular membrane by the bile salt export pump. Following their movement with bile into the lumen of the small intestine, bile acids are almost quantitatively reclaimed in the ileum by the apical sodium-dependent bile acid transporter. The bile acids are shuttled across the enterocyte to the basolateral membrane and effluxed into the portal circulation by the recently indentified heteromeric organic solute transporter, OSTalpha-OSTbeta. In addition to the hepatocyte and enterocyte, subgroups of these bile acid transporters are expressed by the biliary, renal, and colonic epithelium where they contribute to maintaining bile acid homeostasis and play important cytoprotective roles. This article will review our current understanding of the physiological role and regulation of these important carriers.

Medical treatment of primary sclerosing cholangitis: a role for novel bile acids and other (post-)transcriptional modulators?

Primary sclerosing cholangitis (PSC) is a rare chronic cholestatic disease of the liver and bile ducts that is associated with inflammatory bowel disease, generally leads to end-stage liver disease, and is complicated by malignancies of the biliary tree and the large intestine. The pathogenesis of PSC remains enigmatic, making the development of targeted therapeutic strategies difficult. Immunosuppressive and antifibrotic therapeutic agents were ineffective or accompanied by major side effects. Ursodeoxycholic acid (UDCA) has consistently been shown to improve serum liver tests and might lower the risk of colon carcinoma and cholangiocarcinoma by yet unknown mechanisms. Whether "high dose" UDCA improves the long-term prognosis in PSC as suggested by small pilot trials remains to be demonstrated. The present overview discusses potential therapeutic options aside of targeted immunological therapies and UDCA. The C23 bile acid norUDCA has been shown to markedly improve biochemical and histological features in a mouse model of sclerosing cholangitis without any toxic effects. Studies in humans are eagerly being awaited. Nuclear receptors like the farnesoid-X receptor (FXR), pregnane-X receptor (PXR), vitamin D receptor (VDR), and peroxisome-proliferator-activator receptors (PPARs) have been shown to induce expression of diverse carriers and biotransformation enzymes of the intestinal and hepatic detoxification machinery and/or to modulate fibrogenesis. Pros and cons of respective receptor agonists for the future treatment of PSC are discussed in detail. In our view, the novel bile acid norUDCA and agonists of PPARs, VDR, and PXR appear particularly attractive for further studies in PSC.

Intestinal bile acid physiology and pathophysiology

Bile acids (BAs) have a long established role in fat digestion in the intestine by acting as tensioactives, due to their amphipathic characteristics. BAs are reabsorbed very efficiently by the intestinal epithelium and recycled back to the liver via transport mechanisms that have been largely elucidated. The transport and synthesis of BAs are tightly regulated in part by specific plasma membrane receptors and nuclear receptors. In addition to their primary effect, BAs have been claimed to play a role in gastrointestinal cancer, intestinal inflammation and intestinal ionic transport. BAs are not equivalent in any of these biological activities, and structural requirements have been generally identified. In particular, some BAs may be useful for cancer chemoprevention and perhaps in inflammatory bowel disease, although further research is necessary in this field. This review covers the most recent developments in these aspects of BA intestinal biology.

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.

Regional Distribution of Solute Carrier mRNA Expression Along the Human Intestinal Tract

Intestinal absorption of drugs, nutrients, and other compounds is mediated by uptake transporters expressed at the apical enterocyte membrane. These compounds are returned to the intestinal lumen or released into portal circulation by intestinal efflux transporters expressed at apical or basolateral membranes, respectively. One important transporter superfamily, multiple members of which are intestinally expressed, are the solute carriers (SLCs). SLC expression levels may determine the pharmacokinetics of drugs that are substrates of these transporters. In this study we characterize the distribution of 15 human SLC transporter mRNAs in histologically normal biopsies from five regions of the intestine of 10 patients. The mRNA expression levels of CNT1, CNT2, apical sodium-dependent bile acid transporter (ABST), serotonin transporter (SERT), PEPT1, and OCTN2 exhibit marked differences between different regions of the intestine: the first five are predominantly expressed in the small intestine, whereas OCTN2 exhibits strongest expression in the colon. Two transporter mRNAs studied (OCTN1, OATP2B1) are expressed at similar levels in all gut sections. In addition, ENT2 mRNA is present at low levels across the colon, but not in the small intestine. The other six SLC mRNAs studied are not expressed in the intestine. Quantitative knowledge of transporter expression levels in different regions of the human gastrointestinal tract could be useful for designing intestinal delivery strategies for orally administered drugs. Furthermore, changes in transporter expression that occur in pathological states, such as inflammatory bowel disease, can now be defined more precisely by comparison with the expression levels measured in healthy individuals.