Antibiotic-Induced Primary Biles Inhibit SARS-CoV-2 Endoribonuclease Nsp15 Activity in Mouse Gut

The gut microbiome profile of COVID-19 patients was found to correlate with a viral load of SARS-CoV-2, COVID-19 severity, and dysfunctional immune responses, suggesting that gut microbiota may be involved in anti-infection. In order to investigate the role of gut microbiota in anti-infection against SARS-CoV-2, we established a high-throughput in vitro screening system for COVID-19 therapeutics by targeting the endoribonuclease (Nsp15). We also evaluated the activity inhibition of the target by substances of intestinal origin, using a mouse model in an attempt to explore the interactions between gut microbiota and SARS-CoV-2. The results unexpectedly revealed that antibiotic treatment induced the appearance of substances with Nsp15 activity inhibition in the intestine of mice. Comprehensive analysis based on functional profiling of the fecal metagenomes and endoribonuclease assay of antibiotic-enriched bacteria and metabolites demonstrated that the Nsp15 inhibitors were the primary bile acids that accumulated in the gut as a result of antibiotic-induced deficiency of bile acid metabolizing microbes. This study provides a new perspective on the development of COVID-19 therapeutics using primary bile acids.

Current understanding of autophagy in intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy (ICP) is the most common liver disease during pregnancy. Manifested with pruritus and elevation in bile acids, the etiology of ICP is still poorly understood. Although ICP is considered relatively benign for the mother, increased rates of adverse fetal outcomes including sudden fetal demise are possible devastating outcomes associated with ICP. Limited understanding of the underlying mechanisms restricted treatment options and managements of ICP. In recent decades, evolving evidence indicated the significance of autophagy in pregnancy and pregnancy complications. Autophagy is an ancient self-defense mechanism which is essential for cell survival, differentiation and development. Autophagy has pivotal roles in embryogenesis, implantation, and maintenance of pregnancy, and is involved in the orchestration of diverse physiological and pathological cellular responses in patients with pregnancy complications. Recent advances in these research fields provide tantalizing targets on autophagy to improve the care of pregnant women. This review summarizes recent advances in understanding autophagy in ICP and its possible roles in the causation and prevention of ICP.

Bile Acid-Gut Microbiota Axis in Inflammatory Bowel Disease: From Bench to Bedside

Inflammatory bowel disease (IBD) is a chronic, relapsing inflammatory disorder of the gastrointestinal tract, with increasing prevalence, and its pathogenesis remains unclear. Accumulating evidence suggested that gut microbiota and bile acids play pivotal roles in intestinal homeostasis and inflammation. Patients with IBD exhibit decreased microbial diversity and abnormal microbial composition marked by the depletion of phylum Firmicutes (including bacteria involved in bile acid metabolism) and the enrichment of phylum Proteobacteria. Dysbiosis leads to blocked bile acid transformation. Thus, the concentration of primary and conjugated bile acids is elevated at the expense of secondary bile acids in IBD. In turn, bile acids could modulate the microbial community. Gut dysbiosis and disturbed bile acids impair the gut barrier and immunity. Several therapies, such as diets, probiotics, prebiotics, engineered bacteria, fecal microbiota transplantation and ursodeoxycholic acid, may alleviate IBD by restoring gut microbiota and bile acids. Thus, the bile acid–gut microbiota axis is closely connected with IBD pathogenesis. Regulation of this axis may be a novel option for treating IBD.

Gut-Liver Immune Traffic: Deciphering Immune-Pathogenesis to Underpin Translational Therapy

The tight relationship between the gut and liver on embryological, anatomical and physiological levels inspired the concept of a gut-liver axis as a central element in the pathogenesis of gut-liver axis diseases. This axis refers to the reciprocal regulation between these two organs causing an integrated system of immune homeostasis or tolerance breakdown guided by the microbiota, the diet, genetic background, and environmental factors. Continuous exposure of gut microbiome, various hormones, drugs and toxins, or metabolites from the diet through the portal vein adapt the liver to maintain its tolerogenic state. This is orchestrated by the combined effort of immune cells network: behaving as a sinusoidal and biliary firewall, along with a regulatory network of immune cells including, regulatory T cells and tolerogenic dendritic cells (DC). In addition, downregulation of costimulatory molecules on hepatic sinusoids, hepatocytes and biliary epithelial cells as well as regulating the bile acids chain also play a part in hepatic immune homeostasis. Recent evidence also demonstrated the link between changes in the gut microbiome and liver resident immune cells in the progression of cirrhosis and the tight correlation among primary sclerosing cholangitis (PSC) and also checkpoint induced liver and gut injury. In this review, we will summarize the most recent evidence of the bidirectional relationship among the gut and the liver and how it contributes to liver disease, focusing mainly on PSC and checkpoint induced hepatitis and colitis. We will also focus on completed therapeutic options and on potential targets for future treatment linking with immunology and describe the future direction of this research, taking advantage of modern technologies.

Aberrant Gut-To-Brain Signaling in Irritable Bowel Syndrome - The Role of Bile Acids

Functional bowel disorders such as irritable bowel syndrome (IBS) are common, multifactorial and have a major impact on the quality of life of individuals diagnosed with the condition. Heterogeneity in symptom manifestation, which includes changes in bowel habit and visceral pain sensitivity, are an indication of the complexity of the underlying pathophysiology. It is accepted that dysfunctional gut-brain communication, which incorporates efferent and afferent branches of the peripheral nervous system, circulating endocrine hormones and local paracrine and neurocrine factors, such as host and microbially-derived signaling molecules, underpins symptom manifestation. This review will focus on the potential role of hepatic bile acids in modulating gut-to-brain signaling in IBS patients. Bile acids are amphipathic molecules synthesized in the liver, which facilitate digestion and absorption of dietary lipids. They are also important bioactive signaling molecules however, binding to bile acid receptors which are expressed on many different cell types. Bile acids have potent anti-microbial actions and thereby shape intestinal bacterial profiles. In turn, bacteria with bile salt hydrolase activity initiate the critical first step in transforming primary bile acids into secondary bile acids. Individuals with IBS are reported to have altered microbial profiles and modified bile acid pools. We have assessed the evidence to support a role for bile acids in the pathophysiology underlying the manifestation of IBS symptoms.

Mechanisms of Interactions between Bile Acids and Plant Compounds-A Review

Plant compounds are described to interact with bile acids during small intestinal digestion. This review will summarise mechanisms of interaction between bile acids and plant compounds, challenges in in vivo and in vitro analyses, and possible consequences on health. The main mechanisms of interaction assume that increased viscosity during digestion results in reduced micellar mobility of bile acids, or that bile acids and plant compounds are associated or complexed at the molecular level. Increasing viscosity during digestion due to specific dietary fibres is considered a central reason for bile acid retention. Furthermore, hydrophobic interactions are proposed to contribute to bile acid retention in the small intestine. Although frequently hypothesised, no mechanism of permanent binding of bile acids by dietary fibres or indigestible protein fractions has yet been demonstrated. Otherwise, various polyphenolic structures were recently associated with reduced micellar solubility and modification of steroid and bile acid excretion but underlying molecular mechanisms of interaction are not yet fully understood. Therefore, future research activities need to consider the complex composition and cell-wall structures as influenced by processing when investigating bile acid interactions. Furthermore, influences of bile acid interactions on gut microbiota need to be addressed to clarify their role in bile acid metabolism.

Chyme Reinfusion in Intestinal Failure Related to Temporary Double Enterostomies and Enteroatmospheric Fistulas

Some temporary double enterostomies (DES) or entero-atmospheric fistulas (EAF) have high output and are responsible for Type 2 intestinal failure. Intravenous supplementations (IVS) for parenteral nutrition and hydration compensate for intestinal losses. Chyme reinfusion (CR) artificially restores continuity pending surgical closure. CR treats intestinal failure and is recommended by European Society for Clinical Nutrition and Metabolism (ESPEN) and American Society for Parenteral and Enteral Nutrition (ASPEN) when possible. The objective of this study was to show changes in nutritional status, intestinal function, liver tests, IVS needs during CR, and the feasibility of continuing it at home. A retrospective study of 306 admitted patients treated with CR from 2000 to 2018 was conducted. CR was permanent such that a peristaltic pump sucked the upstream chyme and reinfused it immediately in a tube inserted into the downstream intestine. Weight, plasma albumin, daily volumes of intestinal and fecal losses, intestinal nitrogen, and lipid absorption coefficients, plasma citrulline, liver tests, and calculated indices were compared before and during CR in patients who had both measurements. The patients included 185 males and 121 females and were 63 ± 15 years old. There were 37 (12%), 269 (88%) patients with EAF and DES, respectively. The proximal small bowel length from the duodeno-jejunal angle was 108 ± 67 cm (n = 232), and the length of distal small intestine was 117 ± 72 cm (n = 253). The median CR start was 5 d (quartile 25-75%, 2-10) after admission and continued for 64 d (45-95), including 81 patients at home for 47 d (28-74). Oral feeding was exclusive 171(56%), with enteral supplement 122 (42%), or with IVS 23 (7%). Before CR, 211 (69%) patients had IVS for nutrition (77%) or for hydration (23%). IVS were stopped in 188 (89%) 2 d (0-7) after the beginning of CR and continued in 23 (11%) with lower volumes. Nutritional status improved with respect to weight gain (+3.5 ± 8.4%) and albumin (+5.4 ± 5.8 g/L). Intestinal failure was cured in the majority of cases as evidenced by the decrease in intestinal losses by 2096 ± 959 mL/d, the increase in absorption of nitrogen 32 ± 20%, of lipids 43 ± 30%, and the improvement of citrulline 13.1 ± 8.1 µmol/L. The citrulline increase was correlated with the length of the distal intestine. The number of patients with at least one liver test >2N decreased from 84-40%. In cases of Type 2 intestinal failure related to DES or FAE with an accessible and functional distal small bowel segment, CR restored intestinal functions, reduced the need of IVS by 89% and helped improve nutritional status and liver tests. There were no vital complications or infectious diarrhea described to date. CR can become the first-line treatment for intestinal failure related to double enterostomy and high output fistulas.

Mechanism of glucose-lowering by metformin in type 2 diabetes: Role of bile acids

Type 2 diabetes mellitus (T2DM) is an increasingly prevalent chronic condition, characterized by abnormally elevated blood glucose concentrations and, as a consequence, increased risk of micro- and macrovascular complications. Metformin is usually the first-line glucose-lowering medication in T2DM; however, despite being used for more than 60 years, the mechanism underlying the glucose-lowering action of metformin remains incompletely understood. Although metformin reduces hepatic glucose production, there is persuasive evidence that the gastrointestinal tract is crucial in mediating this effect, particularly via secretion of the incretin hormone glucagon-like peptide 1 (GLP-1). It is now well recognized that bile acids, in addition to their established function in fat digestion and absorption, are important regulators of glucose metabolism. Exposure of the small and large intestine to bile acids induces GLP-1 secretion, modulates the composition of the gut microbiota, and reduces postprandial blood glucose excursions in humans with and without T2DM. Metformin reduces intestinal bile acid resorption substantially, such that intraluminal bile acids may, at least in part, account for its glucose-lowering effect. The present review focuses on the conceptual shift in our understanding as to how metformin lowers blood glucose in T2DM, with a particular emphasis on the role of intestinal bile acids.

Microbial Profiles of Cirrhosis in the Human Small Intestine

PURPOSE OF REVIEW

The aim of this review is to summarize the recent literature on associations of small intestinal microbial and bile acid profiles with liver cirrhosis and its complications.

RECENT FINDINGS

Recent studies into the duodenal microbiome of patients with cirrhosis have linked the microbiome to certain etiologies of chronic liver disease as well as complications of cirrhosis. In particular, microbial differences in the duodenum of patients with cirrhosis have been linked to the presence of hepatic encephalopathy and varices. While the fecal microbiome of patients with liver cirrhosis is well characterized, the small intestinal microbiome of cirrhotic patients is an active area of research. This review focuses on the current understanding of the small intestinal microbiome in human cirrhosis as well as future directions of the field.

Signaling from Intestine to the Host: How Bile Acids Regulate Intestinal and Liver Immunity

Primary bile acids (BAs) are generated in the liver as the end products of cholesterol catabolism; they are then conjugated and accumulated in the gallbladder. After a meal ingestion, BAs are reversed into the duodenum to facilitate the lipid absorption. At the intestinal level, the 95% of BAs are reabsorbed and redirected into enterohepatic circulation; indeed only a small amount of them are then subjected to chemical modifications by the intestinal microbiota, which plays a very important role in the generation of secondary bile acids and in regulating host's metabolism and activity of the immune system. Behind their role in nutrients absorption, bile acids act as signaling molecules, activating several receptors, known as bile acid-activated receptors (BARs), including the farnesoid-X-receptor (FXR) and the G protein-coupled bile acid receptor 1 (GPBAR1 or Takeda G-protein receptor 5). Both receptors appear to contribute to maintain the tolerogenic state of the liver and intestine immunity. In particular, FXR and GPBAR1 are highly expressed in cells of innate immunity including intestinal and liver macrophages, dendritic cells, and natural killer T cells. In this chapter, we provide an overview on mechanisms through which FXR and GPBAR1 modulate the signaling between microbiota and intestinal and liver innate immune system. This overview could help to explain beneficial effects exerted by GPBAR1 and FXR agonists in the treatment of metabolic and immuno-mediated diseases.

Nutritional Modulation of Innate Immunity: The Fat-Bile-Gut Connection

Altered nutritional behavior in Western societies has unleashed numerous metabolic disorders, intimately linked to profound disruptions of the immune system. Here we summarize how nutrition modulates innate immunity. We outline recent findings regarding nutrient signaling and we particularly focus on the collateral impact of nutrition on the microbiome and on the bile acid (BA) pool. We discuss how the integration of postprandial signals by the gut microbiota, along with the absorption routes of metabolites, differentially affects immune niches to orchestrate immune responses. Finally, we discuss the potential consequences of these signals in the light of trained immunity. A better understanding of nutrition signaling will permit the optimization of therapeutic and dietary strategies against the arising immune disorders.

Modulation of the intestinal bile acid/farnesoid X receptor/fibroblast growth factor 15 axis improves alcoholic liver disease in mice

Alcoholic liver disease (ALD) is associated with changes in the intestinal microbiota. Functional consequences of alcohol-associated dysbiosis are largely unknown. The aim of this study was to identify a mechanism of how changes in the intestinal microbiota contribute to ALD. Metagenomic sequencing of intestinal contents demonstrated that chronic ethanol feeding in mice is associated with an over-representation of bacterial genomic DNA encoding choloylglycine hydrolase, which deconjugates bile acids in the intestine. Bile acid analysis confirmed an increased amount of unconjugated bile acids in the small intestine after ethanol administration. Mediated by a lower farnesoid X receptor (FXR) activity in enterocytes, lower fibroblast growth factor (FGF)-15 protein secretion was associated with increased hepatic cytochrome P450 enzyme (Cyp)-7a1 protein expression and circulating bile acid levels. Depletion of the commensal microbiota with nonabsorbable antibiotics attenuated hepatic Cyp7a1 expression and reduced ALD in mice, suggesting that increased bile acid synthesis is dependent on gut bacteria. To restore intestinal FXR activity, we used a pharmacological intervention with the intestine-restricted FXR agonist fexaramine, which protected mice from ethanol-induced liver injury. Whereas bile acid metabolism was only minimally altered, fexaramine treatment stabilized the gut barrier and significantly modulated hepatic genes involved in lipid metabolism. To link the beneficial metabolic effect to FGF15, a nontumorigenic FGF19 variant-a human FGF15 ortholog-was overexpressed in mice using adeno-associated viruses. FGF19 treatment showed similarly beneficial metabolic effects and ameliorated alcoholic steatohepatitis.

CONCLUSION

Taken together, alcohol-associated metagenomic changes result in alterations of bile acid profiles. Targeted interventions improve bile acid-FXR-FGF15 signaling by modulation of hepatic Cyp7a1 and lipid metabolism, and reduce ethanol-induced liver disease in mice. (Hepatology 2018;67:2150-2166).

Update on Bile Acid Malabsorption: Finally Ready for Prime Time?

PURPOSE OF REVIEW

To provide an update on the prevalence, pathophysiology, disease associations, and treatment options for bile acid malabsorption (BAM).

RECENT FINDINGS

•Molecular mechanisms-BAs prevent water reabsorption and increase water secretion by intracellular mediators, increasing aquaporin channels and intracellular permeability. •Inflammatory bowel disease-new molecular mechanisms of BAM are identified in patients without ileal disease, including changes in expression of ileal BA transporter and nuclear receptors involved in BA homeostasis. •Microscopic colitis-BAM is one of the mechanisms leading to microscopic colitis. •Diagnostic testing-new diagnostic tests have been launched in the USA (serum C4 and fecal 48-h BA excretion); stimulated FGF19 has higher detection of BAM compared to fasting sample alone. •Treatment-investigational FXR agonists may provide a daily, oral option for treatment of BAM instead of BA sequestrants. There is a greater appreciation of the biological role of bile acids across multiple fields of medicine, including gastrointestinal indications.

Aryl hydrocarbon receptor (AhR) mediated short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid homeostasis in mice

The effects of the most potent aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid (BA) homeostasis was examined in male and female wild-type and AhR-null mice shortly after 4-day exposure, rather than at a later time when secondary non-AhR dependent effects are more likely to occur. TCDD had similar effects on BA homeostasis in male and female mice. TCDD decreased the concentration of total-(Σ) BAs in liver by approximately 50% (all major BA categories except for the non-6,12-OH BAs), without decreasing the expression of the rate limiting BA synthetic enzyme (Cyp7a1) or altering the major BA regulatory pathways (FXR) in liver and intestine. Even though the Σ-BAs in liver were markedly decreased, the Σ-BAs excreted into bile were not altered. TCDD decreased the relative amount of 12-OH BAs (TCA, TDCA, CA, DCA) in bile and increased the biliary excretion of TCDCA and its metabolites (TαMCA, TUDCA); this was likely due to the decreased Cyp8b1 (12α-hydroxylase) in liver. The concentration of Σ-BAs in serum was not altered by TCDD, indicating that serum BAs do not reflect BA status in liver. However, proportions of individual BAs in serum reflected the decreased expression of Cyp8b1. All these TCDD-induced changes in BA homeostasis were absent in AhR-null mice. In summary, through the AhR, TCDD markedly decreases BA concentrations in liver and reduces the 12α-hydroxylation of BAs without altering Cyp7a1 and FXR signaling. The TCDD-induced decrease in Σ-BAs in liver did not result in a decrease in biliary excretion or serum concentrations of Σ-BAs.

Bile acid-microbiota crosstalk in gastrointestinal inflammation and carcinogenesis

Emerging evidence points to a strong association between the gut microbiota and the risk, development and progression of gastrointestinal cancers such as colorectal cancer (CRC) and hepatocellular carcinoma (HCC). Bile acids, produced in the liver, are metabolized by enzymes derived from intestinal bacteria and are critically important for maintaining a healthy gut microbiota, balanced lipid and carbohydrate metabolism, insulin sensitivity and innate immunity. Given the complexity of bile acid signalling and the direct biochemical interactions between the gut microbiota and the host, a systems biology perspective is required to understand the liver-bile acid-microbiota axis and its role in gastrointestinal carcinogenesis to reverse the microbiota-mediated alterations in bile acid metabolism that occur in disease states. An examination of recent research progress in this area is urgently needed. In this Review, we discuss the mechanistic links between bile acids and gastrointestinal carcinogenesis in CRC and HCC, which involve two major bile acid-sensing receptors, farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (TGR5). We also highlight the strategies and cutting-edge technologies to target gut-microbiota-dependent alterations in bile acid metabolism in the context of cancer therapy.

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

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

[Advances in studies of ileal apical sodium-dependent bile acid transporter]

Bile acids play critical roles in the regulation of metabolism and absorption of lipids. The ileal apical sodium-dependent bile acid transporter (ASBT) located at the enterocyte brush border is responsible for the reuptake of bile acids and the maintenance of bile acid homeostasis. Recently, a number of investigations have been made concerning the regulation and control of ASBT and the relationship between ASBT and intestinal inflammation, tumorigenesis, diabetes mellitus and hyperlipemia, which suggests ASBT as a potential therapeutic target of these diseases. In this review, advances in the study of above-mentioned issues were summarized.

Fibroblast growth factor 21 correlates with weight loss after vertical sleeve gastrectomy in adolescents

OBJECTIVE

Vertical sleeve gastrectomy (VSG) results in weight loss and increased bile acids (BA) and fibroblast growth factor 19 (FGF19) levels. FGF21 shares essential cofactors with FGF19, but its physiology early post-VSG has not been assessed.

METHODS

Ten adolescents (17.4 ± 0.5 years and BMI 51.5 ± 2.5 kg/m² ) were enrolled. Fasting and postmeal (100 mL Ensure™) samples (0-120 min) were collected (pre-VSG [V1], 1 [V2], and 3 months [V3] post-VSG) for analysis of BA, FGF19, and FGF21.

RESULTS

Post-VSG subjects lost weight (V2 11.8 ± 0.8 kg; V3 21.9 ± 1.7 kg). BA and FGF19 increased by V2, 143.6% at 30 min and 74.9% at 90 min post-meal, respectively. BA hydrophobicity index also improved by V3, 21.1% at 30 min post-meal. Interestingly, fasting and 120-min post-meal FGF21 levels at V2 were increased by 135.7% and 253.9%, respectively, but then returned to baseline at V3. BA levels correlated with FGF21 at V2 (P = 0.003, r = 0.89), and body weight lost post-VSG correlated with FGF21 levels (V2; P = 0.012, R = 0.82).

CONCLUSIONS

Expected changes were seen in BA and FGF19 biology after VSG in adolescents, but novel changes were seen in correlation between the early postsurgical increase in FGF21 and weight loss, suggesting that FGF21 may play a role in energy balance postoperatively, and further investigation is warranted.

Liver regeneration - mechanisms and models to clinical application

Liver regeneration has been studied for many decades and the mechanisms underlying regeneration of the normal liver following resection or moderate damage are well described. A large number of factors extrinsic (such as bile acids and circulating growth factors) and intrinsic to the liver interact to initiate and regulate liver regeneration. Less well understood, and more clinically relevant, are the factors at play when the abnormal liver is required to regenerate. Fatty liver disease, chronic scarring, prior chemotherapy and massive liver injury can all inhibit the normal programme of regeneration and can lead to liver failure. Understanding these mechanisms could enable the rational targeting of specific therapies to either reduce the factors inhibiting regeneration or directly stimulate liver regeneration. Although animal models of liver regeneration have been highly instructive, the clinical relevance of some models could be improved to bridge the gap between our in vivo model systems and the clinical situation. Likewise, modern imaging techniques such as spectroscopy will probably improve our understanding of whole-organ metabolism and how this predicts the liver's regenerative capacity. This Review describes briefly the mechanisms underpinning liver regeneration, the models used to study this process, and discusses areas in which failed or compromised liver regeneration is clinically relevant.

Bile Diversion in Roux-en-Y Gastric Bypass Modulates Sodium-Dependent Glucose Intestinal Uptake

Gastro-intestinal exclusion by Roux-en-Y gastric bypass (RYGB) improves glucose metabolism, independent of weight loss. Although changes in intestinal bile trafficking have been shown to play a role, the underlying mechanisms are unclear. We performed RYGB in minipigs and showed that the intestinal uptake of ingested glucose is blunted in the bile-deprived alimentary limb (AL). Glucose uptake in the AL was restored by the addition of bile, and this effect was abolished when active glucose intestinal transport was blocked with phlorizin. Sodium-glucose cotransporter 1 remained expressed in the AL, while intraluminal sodium content was markedly decreased. Adding sodium to the AL had the same effect as bile on glucose uptake. It also increased postprandial blood glucose response in conscious minipigs following RYGB. The decrease in intestinal uptake of glucose after RYGB was confirmed in humans. Our results demonstrate that bile diversion affects postprandial glucose metabolism by modulating sodium-glucose intestinal cotransport.

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.

Beyond intestinal soap--bile acids in metabolic control

Over the past decade, it has become apparent that bile acids are involved in a host of activities beyond their classic functions in bile formation and fat absorption. The identification of the farnesoid X receptor (FXR) as a nuclear receptor directly activated by bile acids and the discovery that bile acids are also ligands for the membrane-bound, G-protein coupled bile acid receptor 1 (also known as TGR5) have opened new avenues of research. Both FXR and TGR5 regulate various elements of glucose, lipid and energy metabolism. Consequently, a picture has emerged of bile acids acting as modulators of (postprandial) metabolism. Therefore, strategies that interfere with either bile acid metabolism or signalling cascades mediated by bile acids may represent novel therapeutic approaches for metabolic diseases. Synthetic modulators of FXR have been designed and tested, primarily in animal models. Furthermore, the use of bile acid sequestrants to reduce plasma cholesterol levels has unexpected benefits. For example, treatment of patients with type 2 diabetes mellitus (T2DM) with sequestrants causes substantial reductions in plasma levels of glucose and HbA1c. This Review aims to provide an overview of the molecular mechanisms by which bile acids modulate glucose and energy metabolism, particularly focusing on the glucose-lowering actions of bile acid sequestrants in insulin resistant states and T2DM.

[FXR and its influence on hepatobiliary and gastrointestinal diseases]

Farnesoid X receptor (FXR), a bile acid receptor, serves as a member of the nuclear receptor superfamily. It plays a pivotal role in regulating hepatic inflammation and regeneration. Recent evidence suggests, that FXR and the bile acid are involved in diseases such as cholestatic and inflammatory liver diseases, hepatocellular carcinoma, and bacterial translocation. In this review we discuss the pathophysiology influence of FXR and the bile acid on liver, biliary and gastrointestinal diseases and its mechanisms.

Proinflammatory cytokines and bile acids upregulate ΔNp73 protein, an inhibitor of p53 and p73 tumor suppressors

Gastroesophageal reflux disease (GERD) is the main etiological factor behind the recent rapid increase in the incidence of esophageal adenocarcinoma. During reflux, esophageal cells are exposed to bile at low pH resulting in cellular damage and inflammation, which are known to facilitate cancer development. In this study, we investigated the regulation of p73 isoform, ΔNp73α, in the reflux condition. Previous studies have reported that ΔNp73 exhibits anti-apoptotic and oncogenic properties through inhibition of p53 and p73 proteins. We found that direct exposure of esophageal cells to bile acids in an acidic environment alters the phosphorylation of ΔNp73, its subcellular localization and increases ΔNp73 protein levels. Upregulation of ΔNp73 was also observed in esophageal tissues collected from patients with GERD and Barrett’s metaplasia, a precancerous lesion in the esophagus associated with gastric reflux. c-Abl, p38 MAPK, and IKK protein kinases were identified to interact in the regulation of ΔNp73. Their inhibition with chemotherapeutic agents and siRNA suppresses ΔNp73. We also found that pro-inflammatory cytokines, IL-1β and TNFα, are potent inducers of ΔNp73α, which further enhance the bile acids/acid effect. Combined, our studies provide evidence that gastroesophageal reflux alters the regulation of oncogenic ΔNp73 isoform that may facilitate tumorigenic transformation of esophageal metaplastic epithelium.

Nuclear receptors in regenerating liver and hepatocellular carcinoma

A comprehensive understanding of the pathways underlying hepatocyte turnover and liver regeneration is essential for the development of innovative and effective therapies in the management of chronic liver disease, and the prevention of hepatocellular carcinoma (HCC) in cirrhosis. Nuclear receptors (NRs) are master transcriptional regulators of liver development, differentiation and function. NRs have been implicated in the modulation of hepatocyte priming and proliferation in regenerating liver, chronic hepatitis and HCC development. In this review, we focus on NRs and their pathways regulating hepatocyte proliferation and liver regeneration, with a perspective view on NRs as candidate biomarkers and novel pharmacological targets in the management of liver disease and HCC.

Prolonged stimulation of pancreatic serous secretions by bile and sodium taurocholate in anaesthetized rats

There have been numerous reports that infusion of either natural bile or bile salts into the duodenum evokes a rapid increase in pancreatic secretion through the release of the hormone secretin from the duodenal mucosa. We have extended this observation by the demonstration of an additional late increase in secretion which persisted for many hours and have sought to identify the processes underlying this increase. In anaesthetised rats, infusion of 20 mM taurocholate into the duodenum caused a staircase-like increase in the weight of pancreatic secretion which extended over many hours during which, the HCO[Formula: see text] and protein output of the secretion showed only minimal changes. This effect was also reproduced with intra-duodenal infusion of natural bile which was inferred to act though its taurocholate content. Since the stimulatory action was also obtained with superfusion of taurocholate or natural bile onto the small intestine and by intravenous injection of taurocholate, it was concluded that taurocholate acted by being absorbed into the bloodstream and then by exerting a stimulatory action on the exocrine pancreas. This action was inhibited by puromycin (a protein synthesis inhibitor), by furosemide (a Na( + )/K( + )/2Cl(-) cotransporter inhibitor), though not by SITS (an inhibitor of Cl(-)/HCO[Formula: see text] exchange). The long lasting increase in pancreatic serous secretion would be consistent with the possible activation of gene transcription by taurocholate leading to increased activity of the Na( + )/K( + )/2Cl(-) cotransporter through which the acinar cells increased their secretions.

A novel 3-hydroxysteroid dehydrogenase that regulates reproductive development and longevity

Endogenous small molecule metabolites that regulate animal longevity are emerging as a novel means to influence health and life span. In C. elegans, bile acid-like steroids called the dafachronic acids (DAs) regulate developmental timing and longevity through the conserved nuclear hormone receptor DAF-12, a homolog of mammalian sterol-regulated receptors LXR and FXR. Using metabolic genetics, mass spectrometry, and biochemical approaches, we identify new activities in DA biosynthesis and characterize an evolutionarily conserved short chain dehydrogenase, DHS-16, as a novel 3-hydroxysteroid dehydrogenase. Through regulation of DA production, DHS-16 controls DAF-12 activity governing longevity in response to signals from the gonad. Our elucidation of C. elegans bile acid biosynthetic pathways reveals the possibility of novel ligands as well as striking biochemical conservation to other animals, which could illuminate new targets for manipulating longevity in metazoans.

Molecular mechanisms underlying bile acid-stimulated glucagon-like peptide-1 secretion

BACKGROUND AND PURPOSE

The glucagon-like peptides GLP-1 and GLP-2 are secreted from enteroendocrine L-cells following nutrient ingestion. Drugs that increase activity of the GLP-1 axis are highly successful therapies for type 2 diabetes, and boosting L-cell secretion is a potential strategy for future diabetes treatment. The aim of the present study was to further our understanding of the bile acid receptor GPBA (TGR5), an L-cell target currently under therapeutic exploration.

EXPERIMENTAL APPROACH

GLUTag cells and mixed primary murine intestinal cultures were exposed to bile acids and a specific agonist, GPBAR-A. Secretion was measured using hormone assays and intracellular calcium and cAMP responses were monitored using real-time imaging techniques.

KEY RESULTS

Bile acid-triggered GLP-1 secretion from GLUTag cells was GPBA-dependent, as demonstrated by its abolition following tgr5 siRNA transfection. Bile acids and GPBAR-A increased GLP-1 secretion from intestinal cultures, with evidence for synergy between the effects of glucose and GPBA activation. Elevation of cAMP was observed following GPBA activation in individual GLUTag cells. Direct calcium responses to GPBAR-A were small, but in the presence of the agonist, a subpopulation of cells that was previously poorly glucose-responsive exhibited robust glucose responses. In vivo, increased delivery of bile to more distal regions of the ileum augmented L-cell stimulation.

CONCLUSIONS AND IMPLICATIONS

GPBA signalling in L-cells involves rapid elevation of cAMP, and enhanced calcium and secretory responses to glucose. Modulation of this receptor therapeutically may be an attractive strategy to enhance GLP-1 secretion and achieve better glycaemic control in diabetic patients.

The role of lipophilic bile acids in the development of cirrhotic cardiomyopathy

Marked hemodynamic changes occur in humans and experimental animals with cirrhotic liver disease. In the heart, basal contractility, responsiveness to beta-adrenoceptor activation, and excitation-contraction coupling (ECC) are negatively affected in models of cirrhosis and portal hypertension with portosystemic shunting (PVS), and comprise what has been called cirrhotic cardiomyopathy. These effects are accompanied by elevated circulating levels of bile acids. We investigated whether elevated bile acids act as a myocardial toxicant by exposing cardiac muscle in vitro to bile acids and compared these results with two models of cirrhotic cardiomyopathy with elevated bile acids: CCl4-induced cirrhosis and PVS. Cholic acid, a lipophilic bile acid, produced a decrease in basal cardiac contractility and responsiveness to beta-adrenoceptor activation, both of which appeared to result from altered ECC. beta-Adrenoceptor density and signaling were unaffected. Acutely, ursodeoxycholic acid, a more hydrophilic bile acid, had no effect. Cirrhosis produced a decrease in basal force, depressed beta-adrenoceptor responsiveness, and altered ECC similar to cholic acid. However, cirrhosis also altered beta-adrenoceptor signaling including decreases in cyclic AMP formation, expression of the stimulatory G protein, GS, and beta-adrenoceptor density. Displacement of lipophilic bile acids by chronic administration of ursodeoxycholic acid to rats during the development of cirrhotic cardiomyopathy produced by PVS produced attenuation of the effect on ECC. These results suggest a possible role for lipophilic bile acids in some, but not all of the myocardial consequences of chronic portal vein stenosis and CCl4-induced cirrhosis.

Gut hormones and the brain

The dialogue between gut hormone, bile acids, and the brain plays an important role in energy homeostasis and the onset of Type 2 diabetes mellitus (T2DM). The present review focuses on: (i) bile acid metabolism and the role of bile acids in the regulation of both glucose homeostasis and the control of hypercholesterolemia; (ii) the role of gut hormones in energy homeostasis; and (iii) translation of the pathophysiology of bile acids and gut hormones into clinical practice. Although definitive mechanisms of action of gut hormones and bile acids have not been elucidated completely, these concepts allow us to understand several pharmacological interventions in the treatment of T2DM. Results from further clinical studies with related therapies will help us determine the role of these treatments in the management of energy homeostasis.

Bile-induced secretion of glucagon-like peptide-1: pathophysiological implications in type 2 diabetes?

During the last decades it has become clear that bile acids not only act as simple fat solubilizers, but additionally represent complex hormonal metabolic integrators. Bile acids activate both nuclear receptors (controlling transcription of genes involved in for example bile acid, cholesterol, and glucose metabolism) and the cell surface G protein-coupled receptor TGR5 (modulating energy expenditure in brown fat and muscle cells). It has been shown that TGR5 is expressed in enteroendocrine L cells, which secrete the potent glucose-lowering incretin hormone glucagon-like peptide-1 (GLP-1). Recently it was shown that bile acid-induced activation of TGR5 results in intestinal secretion of GLP-1 and that enhanced TGR5 signaling improves postprandial glucose tolerance in diet-induced obese mice. This Perspectives article presents these novel findings in the context of prior studies on nutrient-induced GLP-1 secretion and outlines the potential implications of bile acid-induced GLP-1 secretion in physiological, pathophysiological, and pharmacological perspectives.

Diversity of bile salts in fish and amphibians: evolution of a complex biochemical pathway

Bile salts are the major end metabolites of cholesterol and are also important in lipid and protein digestion, as well as shaping of the gut microflora. Previous studies had demonstrated variation of bile salt structures across vertebrate species. We greatly extend prior surveys of bile salt variation in fish and amphibians, particularly in analysis of the biliary bile salts of Agnatha and Chondrichthyes. While there is significant structural variation of bile salts across all fish orders, bile salt profiles are generally stable within orders of fish and do not correlate with differences in diet. This large data set allowed us to infer evolutionary changes in the bile salt synthetic pathway. The hypothesized ancestral bile salt synthetic pathway, likely exemplified in extant hagfish, is simpler and much shorter than the pathway of most teleost fish and terrestrial vertebrates. Thus, the bile salt synthetic pathway has become longer and more complex throughout vertebrate evolution. Analysis of the evolution of bile salt synthetic pathways provides a rich model system for the molecular evolution of a complex biochemical pathway in vertebrates.

Mechanisms for increased expression of cholesterol 7alpha-hydroxylase (Cyp7a1) in lactating rats

Cholesterol 7alpha-hydroxylase (Cyp7a1) and the bile acid pool size are increased 2 to 3-fold in lactating postpartum rats. We investigated the interaction of nuclear receptors with the Cyp7a1 proximal promoter and the expression of regulatory signaling pathways in postpartum rats at day 10 (PPd10) versus female controls to identify the mechanisms of increased expression of Cyp7a1, which is maximal at 16 hours. Liver X receptor (LXRalpha) and RNA polymerase II (RNA Pol II) recruitment to Cyp7a1 chromatin were increased 1.5- and 2.5-fold, respectively, at 16 hours on PPd10. Expression of nuclear receptors farnesoid X receptor (FXR), LXRalpha, liver receptor homolog (LRH-1), hepatocyte nuclear factor 4alpha (HNF4alpha), and short heterodimer partner (SHP) messenger RNA (mRNA) and coactivator peroxisome proliferators-activated receptor gamma coactivator-1alpha (PGC-1alpha) mRNA was unchanged in PPd10 versus controls at 16 hours, whereas chicken ovalbumin upstream transcription factor II (COUP-TFII) was decreased 40% at 16 hours. Investigation of a repressive signaling pathway, the c-Jun-N-terminal kinase (JNK) signaling pathway in PPd10 versus controls, showed decreased mRNA expression of hepatocyte growth factor (HGF; decreased 60% at 16 hours) and tyrosine kinase receptor c-Met (decreased 44%-50% at 16 hours), but these were not accompanied by decreased expression of phosphorylated c-Jun. Importantly, expression of fibroblast growth factor 15 (FGF15) mRNA in the ileum was decreased 70% in PPd10 versus controls, whereas phosphorylated mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 (Erk1/2) protein expression in liver was decreased 88% at 16 hours.

CONCLUSION

The increased recruitment of LXRalpha, a Cyp7a1 stimulatory pathway, and decreased expression of FGF15 and phosphorylated Erk1/2, a Cyp7a1 repressive pathway, combined to increase Cyp7a1 expression during lactation.

Circadian dysregulation disrupts bile acid homeostasis

Background

Bile acids are potentially toxic compounds and their levels of hepatic production, uptake and export are tightly regulated by many inputs, including circadian rhythm. We tested the impact of disrupting the peripheral circadian clock on integral steps of bile acid homeostasis.

Methodology/Principal Findings

Both restricted feeding, which phase shifts peripheral clocks, and genetic ablation in Per1^−/−/Per2^−/− (PERDKO) mice disrupted normal bile acid control and resulted in hepatic cholestasis. Restricted feeding caused a dramatic, transient elevation in hepatic bile acid levels that was associated with activation of the xenobiotic receptors CAR and PXR and elevated serum aspartate aminotransferase (AST), indicative of liver damage. In the PERDKO mice, serum bile acid levels were elevated and the circadian expression of key bile acid synthesis and transport genes, including Cyp7A1 and NTCP, was lost. This was associated with blunted expression of a primary clock output, the transcription factor DBP, which transactivates the promoters of both genes.

Conclusions/Significance

We conclude that disruption of the circadian clock results in dysregulation of bile acid homeostasis that mimics cholestatic disease.

Bile acids: chemistry, physiology, and pathophysiology

The family of bile acids includes a group of molecular species of acidic steroids with very peculiar physical-chemical and biological characteristics. They are synthesized by the liver from cholesterol through several complementary pathways that are controlled by mechanisms involving fine-tuning by the levels of certain bile acid species. Although their best-known role is their participation in the digestion and absorption of fat, they also play an important role in several other physiological processes. Thus, genetic abnormalities accounting for alterations in their synthesis, biotransformation and/or transport may result in severe alterations, even leading to lethal situations for which the sole therapeutic option may be liver transplantation. Moreover, the increased levels of bile acids reached during cholestatic liver diseases are known to induce oxidative stress and apoptosis, resulting in damage to the liver parenchyma and, eventually, extrahepatic tissues. When this occurs during pregnancy, the outcome of gestation may be challenged. In contrast, the physical-chemical and biological properties of these compounds have been used as the bases for the development of drugs and as pharmaceutical tools for the delivery of active agents.

Pathogenesis of Barrett esophagus: deoxycholic acid up-regulates goblet-specific gene MUC2 in concert with CDX2 in human esophageal cells

HYPOTHESIS

Bile acid exposure can induce caudal-related homeobox 2 (CDX2) messenger RNA (mRNA) expression, a transcription factor that plays a crucial role in the development of Barrett esophagus. We investigated mucin 2 (MUC2) and CDX2 mRNA expression before and after treatment with deoxycholic acid in 4 human esophageal cell lines.

DESIGN, SETTING, AND PARTICIPANTS

Four human esophageal cell lines-(1) normal squamous cells immortalized by SV40 (Het-1A), (2) adenocarcinoma (SEG-1), and (3 and 4) squamous cell carcinoma (HKESC-1 and HKESC-2)-were exposed in culture for 1 to 24 hours to 100 microM to 1000 microM deoxycholic acid. Total RNA was extracted before and after bile acid treatment and reverse transcribed to complementary DNA.

MAIN OUTCOME MEASURE

MUC2 and CDX2 mRNA expression as determined by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

MUC2 mRNA expression was absent before deoxycholic acid exposure in all 4 cell lines. MUC2 expression increased in a dose- and time-dependent manner with deoxycholic acid in all cell lines. Deoxycholic acid activated MUC2 up-regulation, which correlated directly with CDX2 up-regulation in all 4 cell lines.

CONCLUSIONS

Bile acids up-regulate both intestinal differentiation factor CDX2 and goblet cell-specific gene MUC2 in normal esophageal and cancer cell lines. Further, bile acid-stimulated MUC2 up-regulation correlates directly with CDX2 up-regulation. The simultaneous up-regulation of both CDX2 and MUC2 after bile acid exposure provides molecular evidence of the role of bile acid in the pathogenesis of Barrett esophagus.

The pathogenesis of Barrett's esophagus: secondary bile acids upregulate intestinal differentiation factor CDX2 expression in esophageal cells

INTRODUCTION

Clinical evidence strongly suggests that bile acids are important in the development of Barrett's esophagus, although the mechanism remains unknown. Caudal-related homeobox 2 (CDX2) is a transcription factor recently implicated in early differentiation and maintenance of normal intestinal epithelium and is suggested to play a key role in the pathogenesis of intestinal metaplasia in Barrett's esophagus.

OBJECTIVE

The aim of this study was to investigate the effect of primary and secondary bile acids on CDX2 mRNA expression in human esophageal cells.

METHODS

Human esophageal cells: (1) squamous, immortalized by SV40 (Het-1A); (2) adenocarcinoma (SEG-1); and (3) squamous cell carcinoma (HKESC-1 & HKESC-2), were exposed in cell culture for 1-24 h to 100-1,000 microM deoxycholic, chenodeoxycholic, and glycocholic acids. Total RNA was extracted before and after bile acid treatment and reverse transcribed to cDNA. CDX2 mRNA expression was determined by both quantitative real-time and reverse transcription PCR (RT-PCR).

RESULTS

CDX2 mRNA expression was absent before bile acid exposure in all cell lines. CDX2 expression increased in a dose- and time-dependent fashion with deoxycholic and chenodeoxycholic, but not glycocholic, acid in all four cell lines. The maximal induction of CDX2 expression was seen in SEG-1 adenocarcinoma cells. Expression in Het-1A cells also increased significantly as did expression in HKESC-1,2 cells, although to a lesser extent than in adenocarcinoma.

CONCLUSIONS

These findings show that secondary bile acid stimulation upregulates CDX2 gene expression in both normal and cancer cell lines. They further support the role of bile acids in the pathogenesis of Barrett's esophagus and link the clinical evidence of a high prevalence of luminal bile acids in Barrett's to expression of the gene thought to be responsible for the phenotypic expression of intestinal metaplasia.

Bile salt-dependent efflux of cellular phospholipids mediated by ATP binding cassette protein B4

Human ABCB4 (multidrug resistance [MDR]3 P-glycoprotein) is expressed in the canalicular membrane of the hepatocyte. ABCB4 has been shown to be required for phosphatidylcholine (PC) secretion into the bile and to translocate PC across the plasma membrane. To further investigate the function of ABCB4, we established a cell line stably expressing ABCB4 (human embryonic kidney [HEK]/ABCB4). The efflux of phospholipids from HEK/ABCB4 cells was remarkably increased by the addition of taurocholate. In addition, the cholesterol efflux from HEK/ABCB4 cells was also enhanced in the presence of taurocholate. Light scattering measurements suggested that the taurocholate monomer plays an important role in ABCB4-mediated lipid secretion. On the other hand, the efflux of phospholipids and cholesterol was not mediated by ABCB1 (MDR1) even in the presence of taurocholate. Taurocholate promoted the efflux of phospholipids and cholesterol from HEK/ABCB4 cells more efficiently than glycocholate and cholate. ABCB4-K435M and ABCB4-K1075M, Walker A lysine mutants, did not mediate the phospholipid and cholesterol efflux in the presence of taurocholate, suggesting that ATP hydrolysis is essential for the efflux. Verapamil completely inhibited the taurocholate-dependent efflux of phospholipids and cholesterol from HEK/ABCB4 cells. Mass spectrometry revealed that, in the presence of taurocholate, HEK/ABCB4 cells preferentially secreted PC compared to sphingomyelin. PC vesicles induced cholesterol diffusion from cell membrane, but did not accept cholesterol from ABCB4.

CONCLUSION

ABCB4 mediates the efflux of phospholipids into the canalicular lumen in the presence of bile salts, and plays a crucial role in bile formation and lipid homeostasis.

Analysis of HSD3B7 knockout mice reveals that a 3alpha-hydroxyl stereochemistry is required for bile acid function

Primary bile acids are synthesized from cholesterol in the liver and thereafter are secreted into the bile and small intestine. Gut flora modify primary bile acids to produce secondary bile acids leading to a chemically diverse bile acid pool that is circulated between the small intestine and liver. A majority of primary and secondary bile acids in higher vertebrates have a 3alpha-hydroxyl group. Here, we characterize a line of knockout mice that cannot epimerize the 3beta-hydroxyl group of cholesterol and as a consequence synthesize a bile acid pool in which 3beta-hydroxylated bile acids predominate. This alteration causes death in 90% of newborn mice and decreases the absorption of dietary cholesterol in surviving adults. Negative feedback regulation of bile acid synthesis mediated by the farnesoid X receptor (FXR) is disrupted in the mutant mice. We conclude that the correct stereochemistry of a single hydroxyl group at carbon 3 in bile acids is required to maintain their physiologic and regulatory functions in mammals.

Mechanisms involved in vitamin E transport by primary enterocytes and in vivo absorption

It is generally believed that vitamin E is absorbed along with chylomicrons. However, we previously reported that human colon carcinoma Caco-2 cells use dual pathways, apolipoprotein B (apoB)-lipoproteins and HDLs, to transport vitamin E. Here, we used primary enterocytes and rodents to identify in vivo vitamin E absorption pathways. Uptake of [(3)H]alpha-tocopherol by primary rat and mouse enterocytes increased with time and reached a maximum at 1 h. In the absence of exogenous lipid supply, these cells secreted vitamin E with HDL. Lipids induced the secretion of vitamin E with intermediate density lipoproteins, and enterocytes supplemented with lipids and oleic acid secreted vitamin E with chylomicrons. The secretion of vitamin E with HDL was not affected by lipid supply but was enhanced when incubated with HDL. Microsomal triglyceride transfer protein inhibition reduced vitamin E secretion with chylomicrons without affecting its secretion with HDL. Enterocytes from Mttp-deficient mice also secreted less vitamin E with chylomicrons. In vivo absorption of [(3)H]alpha-tocopherol by mice after poloxamer 407 injection to inhibit lipoprotein lipase revealed that vitamin E was associated with triglyceride-rich lipoproteins and small HDLs containing apoB-48 and apoA-I. These studies indicate that enterocytes use two pathways for vitamin E absorption. Absorption with chylomicrons is the major pathway of vitamin E absorption. The HDL pathway may be important when chylomicron assembly is defective and can be exploited to deliver vitamin E without increasing fat consumption.

Bile acids in combination with low pH induce oxidative stress and oxidative DNA damage: relevance to the pathogenesis of Barrett's oesophagus

BACKGROUND

Barrett's oesophagus is a premalignant condition associated with an increased risk for the development of oesophageal adenocarcinoma (ADCA). Previous studies indicated that oxidative damage contributes to the development of ADCA.

OBJECTIVE

To test the hypothesis that bile acids and gastric acid, two components of refluxate, can induce oxidative stress and oxidative DNA damage.

METHODS

Oxidative stress was evaluated by staining Barrett's oesophagus tissues with different degrees of dysplasia with 8-hydroxy-deoxyguanosine (8-OH-dG) antibody. The levels of 8-OH-dG were also evaluated ex vivo in Barrett's oesophagus tissues incubated for 10 min with control medium and medium acidified to pH 4 and supplemented with 0.5 mM bile acid cocktail. Furthermore, three oesophageal cell lines (Seg-1 cells, Barrett's oesophagus cells and HET-1A cells) were exposed to control media, media containing 0.1 mM bile acid cocktail, media acidified to pH 4, and media at pH 4 supplemented with 0.1 mM bile acid cocktail, and evaluated for induction of reactive oxygen species (ROS).

RESULTS

Immunohistochemical analysis showed that 8-OH-dG is formed mainly in the epithelial cells in dysplastic Barrett's oesophagus. Importantly, incubation of Barrett's oesophagus tissues with the combination of bile acid cocktail and acid leads to increased formation of 8-OH-dG. An increase in ROS in oesophageal cells was detected after exposure to pH 4 and bile acid cocktail.

CONCLUSIONS

Oxidative stress and oxidative DNA damage can be induced in oesophageal tissues and cells by short exposures to bile acids and low pH. These alterations may underlie the development of Barrett's oesophagus and tumour progression.