Luminal Bile Acids Research Articles

Higher Fecal Bile Acid Hydrophobicity Is Associated with Exacerbation of Dextran Sodium Sulfate Colitis in Mice

Increased luminal bile acid hydrophobicity is associated with cytotoxicity and has been suggested to contribute to gut barrier dysfunction. The aim of this study was to compare 2 high-fat diets and a low-fat diet as to whether they modify fecal bile acid profile and hydrophobicity and/or susceptibility to dextran sodium sulfate (DSS) colitis in C57Bl/6J mice. Control and DSS-Control groups received a low-fat control diet [5.5% of total energy (E%) soy oil, 4.5 E% lard], and the DSS-Lard (5.5 E% soy oil, 54.5 E% lard) and DSS-Fish oil (5.5 E% soy oil, 27.2 E% lard and 27.2% menhaden oil) groups received high-fat diets. Feces for bile acid analysis were collected after 3-wk feeding, followed by induction of dextran DSS colitis (2 d 5% DSS in drinking water + 2 d tap water). Fecal bile acid hydrophobicity was elevated 76% in the lard group (P = 0.051) and 122% in the fish oil group (P = 0.001) compared with control, indicating potentially increased cytotoxicity. DSS caused severe colitis symptoms, evaluated as rectal bleeding, whereas all the controls were symptom free. The median symptom scores were: DSS-Control, 2.3 (IQR = 0.6, 3.0); DSS-Lard, 0.3 (IQR = 0, 2.3); and DSS-Fish oil, 2.4 (IQR = 1.9, 2.8). The only differences were DSS-Control vs. control (P < 0.001) and DSS-Fish oil vs. control (P < 0.001). Severity of symptoms in all colitic mice was positively correlated with fecal bile acid hydrophobicity (Spearman's ρ = 0.43; P = 0.028) and fecal deoxycholic acid concentration (Spearman's ρ = 0.39; P = 0.048). These results suggest that luminal bile acid modification, induced by altered dietary fat composition, may alter susceptibility to DSS colitis.

Genetically obese mice do not show increased gut permeability or faecal bile acid hydrophobicity

Gut barrier dysfunction may lead to metabolic endotoxaemia and low-grade inflammation. Recent publications have demonstrated gut barrier dysfunction in obesity induced by a diet high in fat, and a pathogenetic role for luminal bile acids has been proposed. We aimed to investigate whether genetically obese mice develop increased gut permeability and alterations in luminal bile acids on a diet with a regular fat content. We used seven obese male ob/ob mice of C57BL/6J background and ten male wild-type (WT) mice of the same strain. Faeces were collected for bile acid analysis. Intestinal permeability was measured in an Ussing chamber upon euthanasia, using 4 kDa fluorescein isothiocyanate dextran, as per mille (‰, 1/1000) of translocated dextran. We analysed the liver expression of lipopolysaccharide-binding protein (LBP), as well as serum LBP (ELISA). Intestinal permeability was not affected by genetic obesity (jejunum: 0·234 (sem 0·04) ‰ for obese v. 0·225 (sem 0·03) ‰ for WT, P= 0·93; colon: 0·222 (sem 0·06) ‰ for obese v. 0·184 (sem 0·03) ‰ for WT, P= 0·86), nor was liver LBP expression (relative expression: 0·55 (sem 0·08) for obese v. 0·55 (sem 0·13) for WT, P= 0·70). Serum LBP was 2·5-fold higher in obese than in WT mice (P= 0·001). Obese mice had increased daily excretion of total bile acids, but their faecal bile acid hydrophobicity was unchanged. In conclusion, genetic obesity did not impair gut barrier function in mice on a regular chow diet, nor was faecal bile acid hydrophobicity affected.

Bile Acids as Modulators of Gut Microbiota Linking Dietary Habits and Inflammatory Bowel Disease: A Potentially Dangerous Liaison

Bile Acids as Modulators of Gut Microbiota Linking Dietary Habits and Inflammatory Bowel Disease: A Potentially Dangerous Liaison

DIGESTIVE PHYSIOLOGY OF THE PIG SYMPOSIUM: Involvement of gut chemosensing in the regulation of mucosal barrier function and defense mechanisms1,2

Meal ingestion is followed by release of numerous hormones from enteroendocrine cells interspersed among the epithelial cells lining the intestine. Recently, the de-orphanization of G protein-coupled receptor (GPCR)-type nutrient receptors, expressed on the apical membranes of enteroendocrine cells, has suggested a plausible mechanism whereby luminal nutrients trigger the release of gut hormones. Activation of nutrient receptors triggers intracellular signaling mechanisms that promote exocytosis of hormone-containing granules into the submucosal space. Hormones released by foregut enteroendocrine cells include the glucagon-like peptides (GLP) affecting glycemic control (GLP-1) and releasing pro-proliferative, hypertrophy-inducing growth factors (GLP-2). The foregut mucosa, being exposed to pulses of concentrated HCl, is protected by a system of defense mechanisms, which includes epithelial bicarbonate and mucus secretion and augmentation of mucosal blood flow. We have reported that luminal co-perfusion of AA with nucleotides in anesthetized rats releases GLP-2 into the portal vein, associated with increased bicarbonate and mucus secretion and mucosal blood flow. The GLP-2 increases bicarbonate secretion via release of vasoactive intestinal peptide (VIP) from myenteric nerves. Luminal bile acids also release gut hormones due to activation of the bile-acid receptor known as G Protein-Coupled Receptor (GPR) 131, G Protein Bile Acid Receptor (GPBAR) 1, or Takeda G Protein-Coupled Receptor (TGR) 5, also expressed on enteroendocrine cells. The GLP are metabolized by dipeptidyl peptidase IV (DPPIV), an enzyme of particular interest to pharmaceutical, because its inhibition increases plasma concentrations of GLP-1 to treat diabetes. We have also reported that DPPIV inhibition enhances the secretory effects of nutrient-evoked GLP-2. Understanding the release mechanism and the metabolic pathways of gut hormones is of potential utility to the formulation of feedstuff additives that, by increasing nutrient absorption due to increased mucosal mass, can increase yields.

A novel mechanism for gut barrier dysfunction by dietary fat: epithelial disruption by hydrophobic bile acids

Impairment of gut barrier is associated with a fat-rich diet, but mechanisms are unknown. We have earlier shown that dietary fat modifies fecal bile acids in mice, decreasing the proportion of ursodeoxycholic acid (UDCA) vs. deoxycholic acid (DCA). To clarify the potential role of bile acids in fat-induced barrier dysfunction, we here investigated how physiological concentrations of DCA and UDCA affect barrier function in mouse intestinal tissue. Bile acid experiments were conducted in vitro in Ussing chambers using 4- and 20-kDa FITC-labeled dextrans. Epithelial integrity and inflammation were assayed by histology and Western blot analysis for cyclooxygenase-2. LPS was studied in DCA-induced barrier dysfunction. Finally, we investigated in a 10-wk in vivo feeding trial in mice the barrier-disrupting effect of a diet containing 0.1% DCA. DCA disrupted epithelial integrity dose dependently at 1-3 mM, which correspond to physiological concentrations on a high-fat diet. Low-fat diet-related concentrations of DCA had no effect. In vivo, the DCA-containing diet increased intestinal permeability 1.5-fold compared with control (P = 0.016). Hematoxylin-eosin staining showed a clear disruption of the epithelial barrier by 3 mM DCA in vitro. A short-term treatment by DCA did not increase cyclooxygenase-2 content in colon preparations. UDCA did not affect barrier function itself, but it ameliorated DCA-induced barrier disruption at a 0.6 mM concentration. LPS had no significant effect on barrier function at 0.5-4.5 μg/ml concentrations. We suggest a novel mechanism for barrier dysfunction on a high-fat diet involving the effect of hydrophobic luminal bile acids.

Connecting dysbiosis, bile-acid dysmetabolism and gut inflammation in inflammatory bowel diseases

ObjectiveGut microbiota metabolises bile acids (BA). As dysbiosis has been reported in inflammatory bowel diseases (IBD), we aim to investigate the impact of IBD-associated dysbiosis on BA metabolism and its...

Dipeptidyl peptidase IV inhibition potentiates amino acid- and bile acid-induced bicarbonate secretion in rat duodenum

Intestinal endocrine cells release gut hormones, including glucagon-like peptides (GLPs), in response to luminal nutrients. Luminal L-glutamate (L-Glu) and 5'-inosine monophosphate (IMP) synergistically increases duodenal HCO3- secretion via GLP-2 release. Since L cells express the bile acid receptor TGR5 and dipeptidyl peptidase (DPP) IV rapidly degrades GLPs, we hypothesized that luminal amino acids or bile acids stimulate duodenal HCO3- secretion via GLP-2 release, which is enhanced by DPPIV inhibition. We measured HCO3- secretion with pH and CO2 electrodes using a perfused rat duodenal loop under isoflurane anesthesia. L-Glu (10 mM) and IMP (0.1 mM) were luminally coperfused with or without luminal perfusion (0.1 mM) or intravenous (iv) injection (3 μmol/kg) of the DPPIV inhibitor NVP728. The loop was also perfused with a selective TGR5 agonist betulinic acid (BTA, 10 μM) or the non-bile acid type TGR5 agonist 3-(2-chlorophenyl)-N-(4-chlorophenyl)-N,5-dimethylisoxazole-4-carboxamide (CCDC; 10 μM). DPPIV activity visualized by use of the fluorogenic substrate was present on the duodenal brush border and submucosal layer, both abolished by the incubation with NVP728 (0.1 mM). An iv injection of NVP728 enhanced L-Glu/IMP-induced HCO3- secretion, whereas luminal perfusion of NVP728 had no effect. BTA or CCDC had little effect on HCO3- secretion, whereas NVP728 iv markedly enhanced BTA- or CCDC-induced HCO3- secretion, the effects inhibited by a GLP-2 receptor antagonist. Coperfusion of the TGR5 agonist enhanced L-Glu/IMP-induced HCO3- secretion with the enhanced GLP-2 release, suggesting that TGR5 activation amplifies nutrient sensing signals. DPPIV inhibition potentiated luminal L-Glu/IMP-induced and TGR5 agonist-induced HCO3- secretion via a GLP-2 pathway, suggesting that the modulation of the local concentration of the endogenous secretagogue GLP-2 by luminal compounds and DPPIV inhibition helps regulate protective duodenal HCO3- secretion.

Intestinal Epithelial Toll-Like Receptor 4 Regulates Goblet Cell Development and Is Required for Necrotizing Enterocolitis in Mice

Little is known about factors that regulate intestinal epithelial differentiation; microbial recognition receptors such as Toll-like receptor (TLR)4 might be involved. We investigated whether intestinal TLR4 regulates epithelial differentiation and is involved in development of necrotizing enterocolitis (NEC) of the immature intestine. Mice with conditional disruption of TLR4 in the intestinal epithelium and TLR4 knockout (TLR4(-/-)) mice were generated by breeding TLR4(loxp/loxp) mice with villin-cre and Ella-cre, respectively. Enterocytes that did not express or overexpressed TLR4 were created by lentiviral or adenoviral transduction. Intestinal organoids were cultured on tissue matrices. Bile acids were measured by colorimetric assays, and microbial composition was determined by 16S pyrosequencing. NEC was induced in 7- to 10-day-old mice by induction of hypoxia twice daily for 4 days. TLR4(-/-) mice and mice with enterocyte-specific deletion of TLR4 were protected from NEC; epithelial differentiation into goblet cells was increased via suppressed Notch signaling in the small intestinal epithelium. TLR4 also regulates differentiation of goblet cells in intestinal organoid and enterocyte cell cultures; differentiation was increased on deletion of TLR4 and restored when TLR4 was expressed ectopically. TLR4 signaling via Notch was increased in intestinal tissue samples from patients with NEC, and numbers of goblet cells were reduced. 16S pyrosequencing revealed that wild-type and TLR4-deficient mice had similar microbial profiles; increased numbers of goblet cells were observed in mice given antibiotics. TLR4 deficiency reduced levels of luminal bile acids in vivo, and addition of bile acids to TLR4-deficient cell cultures prevented differentiation of goblet cells. TLR4 signaling and Notch are increased in intestinal tissues of patients with NEC and required for induction of NEC in mice. TLR4 prevents goblet cell differentiation, independently of the microbiota. Bile acids might initiate goblet cell development.

Dietary fish oil increases fat absorption and fecal bile acid content without altering bile acid synthesis in 20-d-old weanling rats following massive ileocecal resection

Dietary fish oil (FO) was reported to lower fecal fat excretion in a weanling rat model of short bowel syndrome (SBS) after ileocecal resection (ICR), and to induce changes in secretion and synthesis of bile acid (BA) in adults. We hypothesized that dietary FO, as compared with corn oil (CO), increases intestinal fat absorption in weanling SBS rats in part due to increased hepatic BA synthesis and luminal BA concentrations. After undergoing ICR, 20-d-old rats were fed ad lib for 7 d with a CO or FO diet containing 5% sucrose polybehenate (SPB), a marker for dietary fat absorption. Fecal fatty acid, fecal and intestine luminal BA, liver mRNA expressions of cholesterol 7α-hydroxylase (Cyp7α1) and sterol-12α-hydroxylase (Cyp8β1), and serum 7α-hydroxy-4-cholesten-3-1 (7αC4) levels were determined. As compared with CO-ICR rats, FO-ICR rats had higher intestinal absorption of total fat and most individual fatty acids. Although the BA content per gram of dry stool was increased in FO-ICR rats, there were no differences between groups for the BA content in remnant jejunum, liver mRNA expression of BA biosynthetic enzymes, Cyp7α1 and Cyp8β1, or serum 7αC4, a marker for BA synthesis. Dietary FO increases dietary fat absorption without increasing hepatic BA synthesis in weanling SBS rats.

Apical sodium-dependent bile acid transporter upregulation is associated with necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is the most common gastrointestinal emergency of premature infants. Previously, we showed that luminal bile acids (BAs) are increased and correlated with disease development and that the apical sodium-dependent BA transporter (ASBT), which transports BAs from the ileal lumen into enterocytes, is upregulated in rats with NEC. We hypothesized that intraenterocyte, rather than luminal, BAs are associated with NEC and that upregulation of ASBT may be a mechanism by which this occurs. Neonatal rats with or without the ASBT inhibitor SC-435, mice in which ASBT was knocked out, and mice that overproduce BAs were subjected to the NEC protocol. Disease development, ASBT, and the farnesoid X receptor protein, along with luminal and intraenterocyte BA levels, were assessed. In addition, ileal sections from premature infants with and without NEC were examined for ASBT via immunohistology and real-time PCR. When BAs were not transported into enterocytes (rats given SC-435 and ASBT knockout mice), severity and incidence of NEC were reduced. In contrast, in mice that overproduce BAs, ASBT was elevated, intraenterocyte BAs were increased, and disease development was increased. ASBT staining was more intense on the apical membrane of ileal enterocytes from premature infants with NEC than premature infants with non-NEC diagnoses. In addition, ASBT mRNA levels were significantly higher in infants with NEC. These data show that accumulation of intraenterocyte BAs contributes to disease development, elevated ASBT increases disease severity in experimental models of NEC, and ASBT is elevated in human NEC. These data confirm that BAs and upregulation of ASBT play a crucial role in NEC pathogenesis and suggest that inhibition of ASBT could be utilized as a therapeutic modality against this disease.

Conditional Gata4 deletion in mice induces bile acid absorption in the proximal small intestine

Background and aimsThe transcription factor GATA4 is expressed throughout most of the small intestine except distal ileum, and restricts expression of the apical sodium-dependent bile acid transporter (ASBT), the rate-limiting...

Mutation screening of apical sodium-dependent bile acid transporter (SLC10A2): novel haplotype block including six newly identified variants linked to reduced expression

The apical sodium-dependent bile acid transporter (SLC10A2) plays a key role in the reabsorption of luminal bile acids into the enterohepatic circulation. Rare variations in SLC10A2 have been reported to be associated with Crohn's disease, primary bile acid malabsorption and familial hypertriglyceridemia; however, variants associated with reduced SLC10A2 expression have not been reported to date. In this study, we have performed a sequence analysis of SLC10A2 using genomic DNA of 93 individuals. A new haplotype structure was identified including ten variants with complete linkage disequilibrium (LD' = 1.0, r (2) = 1.0) of which six polymorphisms were novel. The sequence variants were confirmed in three independent cohorts (n = 1,290) by a recently established MALDI-TOF MS iPLEX assay. Remarkably, haplotype carriers with the minor allele exhibited significant reduced ileal SLC10A2 expression on mRNA levels (2.6-fold, P = 0.0009) and protein levels (2.4-fold, P = 0.0157). In future studies a single tag SNP selected of this haplotype block will provide reliable genetic testing to investigate systemically the influence of the SLC10A2 haplotype for disease susceptibility and/or drug response.

29. Bacterial Dependent up Regulation of Ileal Bile Acid Binding Protein and Bile Acid Transporters Is FXR-Mediated Following Ileo-Cecal Resection (ICR)

Enteric microbiota may exacerbate chronic inflammation in patients with Crohn’s disease. In these patients, recurrent disease often results in surgical resection of the terminal ileum, disrupting the normal enterohepatic circulation of bile acids. The ability of the colon to up regulate the molecular mechanisms required for bile acid reclamation following ICR helps prevent colonic mucosa from chronic injury due to increased luminal bile acid content. In this study, we hypothesized that enteric microbiota are required for luminal bile salts to up regulate FXR-mediated bile acid transport in the colon.

The Pathogenesis of Barrett’s Esophagus: Secondary Bile Acids Upregulate Intestinal Differentiation Factor CDX2 Expression in Esophageal Cells

IntroductionClinical 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. ObjectiveThe aim of this study was to investigate the effect of primary and secondary bile acids on CDX2 mRNA expression in human esophageal cells. MethodsHuman 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 100–1,000μm 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). ResultsCDX2 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. ConclusionsThese 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.

Role of bile in pathogenesis of indomethacin-induced enteropathy

Ingestion of non-steroidal anti-inflammatory drugs (NSAIDs) causes an enteropathy. The pathogenesis involves biochemical initiation of intestinal mucosal damage due to NSAID-induced inhibition of cyclooxygenase and the topical effects of these drugs. These effects lead to increased intestinal permeability and inflammation. Luminal bile acids play a controversial role in the damage produced by these drugs. The aim of this study was to determine the role of bile in producing the enteropathy caused by indomethacin, an NSAID commonly used in toxicity studies. Sprague-Dawley rats were subjected to bile duct ligation. Twenty-four hours later, they were dosed with indomethacin. Intestinal permeability ((51)Cr-EDTA) and inflammation (faecal calprotectin) were measured in the animals at various time periods after the dose. Intestinal permeability was significantly higher in rats 1-6 h after dosing with indomethacin, but not at 24-29 h or day 4, when compared with corresponding values for control animals. Excretion of faecal calprotectin was elevated in the indomethacin-treated rats. The drug-treated animals showed no evidence of ulceration when they were sacrificed 29 h or a week after the dose of indomethacin. Bile acids per se did not affect intestinal permeability or faecal excretion of calprotectin, when given along with indomethacin or its vehicle. We conclude that macroscopic small bowel damage does not occur with indomethacin if bile is excluded, despite the induction of permeability and inflammation. This study highlights the importance of luminal factors, such as bile, in producing indomethacin-induced ulceration in the rat small intestine.

Bile Acids Induce Ileal Damage During Experimental Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is the most common gastrointestinal emergency of premature infants. While the effect of bile acids (BAs) on intestinal mucosal injury is known, we investigated the contribution of BAs during the development of NEC in neonatal rats. Premature rats were fed with cow's milk-based formula and subjected to asphyxia and cold stress to develop NEC. Jejunal and ileal luminal BAs, portal blood BAs, and messenger RNA and protein for the apical sodium-dependent bile acid transporter, the ileal bile acid binding protein, and the heteromeric organic solute transporter (Ostalpha/Ostbeta)were evaluated. Ileal luminal BAs levels were increased significantly during disease development and the removal of ileal BAs significantly decreased the incidence and severity of disease. Furthermore, when NEC was reduced via treatment with epidermal growth factor (EGF), BA levels were reduced significantly. Jejunal luminal BA levels were similar between animals with NEC and controls, but portal/ileal luminal BA ratios were decreased significantly in animals with NEC. The apical sodium-dependent bile acid transporter was up-regulated at the site of injury in animals with NEC and decreased after EGF treatment; however, the ileal bile acid binding protein was up-regulated only in the NEC and EGF group. Ostalpha/Ostbeta expression was low in all groups, and only slightly increased in the NEC group. These data strongly suggest that BAs play a role in the development of ileal damage in experimental NEC and that alterations in BA transport in the neonatal ileum may contribute to disease development.

Cholesterol Metabolism Is Affected by Calcium Phosphate Supplementation in Humans

Dietary calcium and phosphate precipitate in the small intestine to form insoluble amorphous calcium phosphate (ACP). The ability of ACP to bind and inactivate luminal bile acids might have an effect on cholesterol metabolism. To test this hypothesis, a placebo-controlled, double-blind, crossover study with pentacalcium hydroxy-triphosphate supplementation (CaP; 1.0 g elemental calcium) was conducted in 31 young healthy volunteers. The CaP was incorporated into bread. Serum cholesterol concentrations were lower after 4 wk of supplementation than after 4 wk of placebo (4.36 vs. 4.60 mmol/L; P = 0.008). Serum LDL cholesterol and the ratio of LDL:HDL cholesterol also tended to be lower after CaP supplementation than after placebo (-5.6%, P = 0.083 and -5.4%, P < 0.062, respectively). The participants' fat and cholesterol intakes and fecal fat excretion did not differ in the 2 periods. Although the analysis of fecal samples showed no difference in the excretion of total neutral sterols (sum of cholesterol and its transformation products), the excretion of cholesterol itself increased (9.64 vs. 5.80 micromol/g dry matter; P = 0.025; n = 25), whereas the excretion of the metabolite coprostanol decreased (18.5 vs. 21.0 mumol/g dry matter; P = 0.025; n = 25) in the CaP period. Bile acid excretion increased during the CaP period compared with the placebo period (25.4 vs. 22.9 micromol/g dry matter; P = 0.003). The observed beneficial effects on cholesterol metabolism are not the result of an increased excretion of cholesterol, but might be explained by an increased bile acid excretion and a subsequent regeneration of bile acids from endogenous cholesterol in the liver.

Cholesterol modulates human intestinal sodium-dependent bile acid transporter

Bile acids are efficiently absorbed from the intestinal lumen via the ileal apical sodium-dependent bile acid transporter (ASBT). ASBT function is essential for maintenance of cholesterol homeostasis in the body. The molecular mechanisms of the direct effect of cholesterol on human ASBT function and expression are not entirely understood. The present studies were undertaken to establish a suitable in vitro experimental model to study human ASBT function and its regulation by cholesterol. Luminal membrane bile acid transport was evaluated by the measurement of sodium-dependent 3H-labeled taurocholic acid (3H-TC) uptake in human intestinal Caco-2 cell monolayers. The relative abundance of human ASBT (hASBT) mRNA was determined by real-time PCR. Transient transfection and luciferase assay techniques were employed to assess hASBT promoter activity. Caco-2 cell line was found to represent a suitable model to study hASBT function and regulation. 25-Hydroxycholesterol (25-HCH; 2.5 microg/ml for 24 h) significantly inhibited Na(+)-dependent 3H-TC uptake in Caco-2 cells. This inhibition was associated with a 50% decrease in the V(max) of the transporter with no significant changes in the apparent K(m). The inhibition in hASBT activity was associated with reduction in both the level of hASBT mRNA and its promoter activity. Our data show the inhibition of hASBT function and expression by 25-HCH in Caco-2 cells. These data provide novel evidence for the direct regulation of human ASBT function by cholesterol and suggest that this phenomenon may play a central role in cholesterol homeostasis.

Omega-3 fatty acids in colorectal cancer prevention.

Omega-3 fatty acids in colorectal cancer prevention.

Inhibition of nitric oxide synthesis potentiates the colonic permeability increase triggered by luminal bile acids.

Experiments were performed in anaesthetized rats to clarify the role of nitric oxide (NO) in the control of colonic permeability. Colonic luminal pressure, the transmucosal potential difference (PD) and the clearance of [3H] mannitol and [14C] urea from blood to lumen were measured. NO synthesis was blocked with Nomega-nitro-L-arginine (L-NNA) i.v. and mucosal permeability was increased by deoxycholic acid (DCA, 4 mm). The involvement of histamine in the response was studied by giving the histamine H1 receptor blocker pyrilamine. In proximal colon, L-NNA per se increased luminal pressure and PD but had no significant effect on clearance. DCA per se increased luminal pressure, had no significant effect on PD, but increased mannitol and urea clearance and the clearance ratio. L-NNA and pyrilamine both blocked the luminal pressure effect of DCA but L-NNA had no significant effect on the clearance response to DCA. In distal colon, L-NNA per se had no significant effect on pressure and clearance, but increased PD like in proximal colon. DCA had no significant effect on luminal pressure, but markedly reduced PD and increased both clearance and clearance ratio. In this segment, L-NNA significantly potentiated the clearance response to DCA, and further increased clearance ratio to a value not significantly different from unity (1.00 +/- 0.05). The data suggest that in vivo, moderate concentrations of bile acids increase colonic permeability in rats via a mechanism that is inhibited by NO in distal but not in proximal colon. In distal colon, NO may contribute to the maintenance of epithelial barrier function.