TGR5-mediated bile acid sensing controls glucose homeostasis

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.

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.

Bile acid receptor TGR5 agonism induces NO production and reduces monocyte adhesion in vascular endothelial cells

OBJECTIVE

TGR5 is a G-protein-coupled receptor for bile acids. So far, little is known about the function of TGR5 in vascular endothelial cells.

APPROACH AND RESULTS

In bovine aortic endothelial cells, treatment with a bile acid having a high affinity to TGR5, taurolithocholic acid (TLCA), significantly increased NO production. This effect was abolished by small interfering RNA-mediated depletion of TGR5. TLCA-induced NO production was also observed in human umbilical vein endothelial cells measured via intracellular cGMP accumulation. TLCA increased endothelial NO synthase(ser1177) phosphorylation in human umbilical vein endothelial cells. This response was accompanied by increased Akt(ser473) phosphorylation and intracellular Ca(2+). Inhibition of these signals significantly decreased TLCA-induced NO production. We next examined whether TGR5-mediated NO production affects inflammatory responses of endothelial cells. In human umbilical vein endothelial cells, TLCA significantly reduced tumor necrosis factor-α-induced adhesion of monocytes, vascular cell adhesion molecule-1 expression, and activation of nuclear factor-κB. TLCA also inhibited lipopolysaccharide-induced monocyte adhesion to mesenteric venules in vivo. These inhibitory effects of TLCA were abrogated by NO synthase inhibition.

CONCLUSIONS

TGR5 agonism induces NO production via Akt activation and intracellular Ca(2+) increase in vascular endothelial cells, and this function inhibits monocyte adhesion in response to inflammatory stimuli.

Novel therapeutic targets for nonalcoholic fatty liver disease

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) is a serious public health problem. It is now estimated to affect 30% of adults and about 10% of children in the U.S. Hispanics are disproportionably affected with not only higher rates of NAFLD but also more severe disease. Treatment options are currently limited.

AREAS COVERED

In this review, we will focus on a series of novel findings related to the pathobiology of liver damage in nonalcoholic steatohepatitis (NASH) that are attractive targets for development of novel therapeutic strategies for human NASH. In particular, we will discuss four different areas due to their novelty and growing importance including microparticles, the inflammasomes, gut-liver axis and dietary lipids.

EXPERT OPINION

There is an urgent need to develop novel safe and effective therapies for the growing NAFLD epidemic. The data discussed in this article provide strong rational to think out of the box when considering novel therapeutic targets for patients with NAFLD.

Nuclear receptors, bile acids and cholesterol homeostasis series - bile acids and pregnancy

Bile acids have been traditionally thought of as having an important role in fat emulsification. It is now emerging that they act as important signalling molecules that not only autoregulate their own synthesis but also influence lipid and glucose metabolism. Although, the mechanisms that underlie the regulation of bile acid homeostasis have been well characterised in normal physiology, the impact of pregnancy on bile acid regulation is still poorly understood. This review summarises the main regulatory mechanisms underlying bile acid homeostasis and discusses how pregnancy, a unique physiological state, can modify them. The fetoplacental adaptations that protect against fetal bile acid toxicity are reviewed. We highlight the importance of bile acid regulation during gestation by discussing the liver disease of pregnancy, intrahepatic cholestasis of pregnancy (ICP) and how genetic, endocrine and environmental factors contribute to the disease aetiology at a cellular and molecular level.

Epigenomic regulation of bile acid metabolism: emerging role of transcriptional cofactors

The traditional role of bile acids is to simply facilitate absorption and digestion of lipid nutrients, but bile acids also act as endocrine signaling molecules that activate nuclear and membrane receptors to control integrative metabolism and energy balance. The mechanisms by which bile acid signals are integrated to regulate target genes are, however, largely unknown. Recently emerging evidence has shown that transcriptional cofactors sense metabolic changes and modulate gene transcription by mediating reversible epigenomic post-translational modifications (PTMs) of histones and chromatin remodeling. Importantly, targeting these epigenomic changes has been a successful approach for treating human diseases, especially cancer. Here, we review emerging roles of transcriptional cofactors in the epigenomic regulation of liver metabolism, especially focusing on bile acid metabolism. Targeting PTMs of histones and chromatin remodelers, together with the bile acid-activated receptors, may provide new therapeutic options for bile acid-related disease, such as cholestasis, obesity, diabetes, and entero-hepatic cancers.

Roux-en-Y gastric bypass normalizes the blunted postprandial bile acid excursion associated with obesity

BACKGROUND

Bile acids (BAs) are nutrient-responsive hormones that modulate energy balance through cell surface and nuclear receptors. Postprandial plasma BAs have been found to be decreased in obesity.

OBJECTIVE

We aimed to determine whether meal-stimulated circulating BA levels are altered by Roux-en-Y gastric bypass (RYGB), an operation that modifies the neurohumoral determinants of food intake and energy expenditure to cause significant and durable weight loss.

DESIGN

Longitudinal study measuring fasting and postprandial plasma BAs before and after RYGB.

SUBJECTS

Five obese surgical patients and eight lean controls underwent frequent blood sampling after a standard liquid meal. Obese subjects were also tested at 1, 4 and 40 weeks after RYGB. Primary and secondary circulating BAs, as well as their glycine and taurine conjugates, were measured via reverse-phase high-performance liquid chromatography/mass spectroscopy.

RESULTS

We found that postprandial excursion of conjugated BAs was 52.4% lower in obese than in lean individuals by area-under-the-curve (AUC) analysis (378 vs 793 μmol min l(-1), respectively, P<0.05). By 40 weeks after RYGB, the meal-induced rise in conjugated BAs increased by 55.5% to the level of healthy lean controls (378 pre-op vs 850 μmol min l(-) post-op by AUC analyses, P<0.05). In contrast, postprandial concentrations of unconjugated BAs were similar in lean and obese individuals and were not affected by surgery.

CONCLUSION

In light of the growing evidence that BAs have key roles in glucose, lipid and energy homeostasis, the observation that RYGB normalizes the blunted postprandial circulating BA response in obesity suggests that BAs may contribute to the improvement in meal-related physiology seen after RYGB. Further studies are warranted to examine this hypothesis and to determine the degree to which an augmented BA response to nutrient ingestion may mediate the increased incretin response, brown adipose tissue activation and thermic effect of feeding that has been observed after this operation.

From gut changes to type 2 diabetes remission after gastric bypass surgeries

Increasing evidence suggests that the gut may influence the host's metabolism and ultimately change the outcomes of type 2 diabetes mellitus (T2DM). We review the evidence on the relationship between the gut and T2DM remission after gastric bypass surgery, and discuss the potential mechanisms underlying the above relationship: gut anatomical rearrangement, microbial composition changes, altered gut cells, and gut hormone modulation. However, the exact changes and their relative importance in the metabolic improvements after gastric bypass surgery remain to be further clarified. Elucidating the precise metabolic mechanisms of T2DM resolution after bypass surgery will help to reveal the molecular mechanisms of pathogenesis, and facilitate the development of novel diagnoses and preventative interventions for this common disease.

FXR agonist INT-747 upregulates DDAH expression and enhances insulin sensitivity in high-salt fed Dahl rats

Aims

Genetic and pharmacological studies have shown that impairment of the nitric oxide (NO) synthase (NOS) pathway is associated with hypertension and insulin-resistance (IR). In addition, inhibition of NOS by the endogenous inhibitor, asymmetric dimethylarginine (ADMA), may also result in hypertension and IR. On the other hand, overexpression of dimethylarginine dimethylaminohydrolase (DDAH), an enzyme that metabolizes ADMA, in mice is associated with lower ADMA, increased NO and enhanced insulin sensitivity. Since DDAH carries a farnesoid X receptor (FXR)-responsive element, we aimed to upregulate its expression by an FXR-agonist, INT-747, and evaluate its effect on blood pressure and insulin sensitivity.

Methods and Results

In this study, we evaluated the in vivo effect of INT-747 on tissue DDAH expression and insulin sensitivity in the Dahl rat model of salt-sensitive hypertension and IR (Dahl-SS). Our data indicates that high salt (HS) diet significantly increased systemic blood pressure. In addition, HS diet downregulated tissue DDAH expression while INT-747 protected the loss in DDAH expression and enhanced insulin sensitivity compared to vehicle controls.

Conclusion

Our study may provide the basis for a new therapeutic approach for IR by modulating DDAH expression and/or activity using small molecules.

Metabolic surgery for type 2 diabetes: efficacy and risks

PURPOSE OF REVIEW

Bariatric surgery has emerged as a highly effective treatment for obesity and is increasingly recognized to have benefits for glycemic management in patients with pre-existing type 2 diabetes mellitus (T2DM), and for diabetes prevention. This article will review the efficacy and mechanisms of metabolic surgery for the treatment of T2DM.

RECENT FINDINGS

Recent small randomized studies with 1-2 years of follow-up begin to provide level 1 data of the effects of surgical procedures compared to medical management on glycemic control. Physiology studies using sophisticated metabolic techniques to evaluate insulin secretion, action, and entero-endocrine changes in patients preoperatively and postoperatively have shed light on the mechanisms that lead to changes in glycemia observed after bariatric surgery.

SUMMARY

Understanding the efficacy and physiologic effects of bariatric surgery on metabolism will help guide patient management in the context of a growing epidemic of obesity and T2DM, and may also ultimately enable us to develop less invasive but equally effective therapeutic strategies for weight loss and metabolic control.

The effect of a bile acid sequestrant on glucose metabolism in subjects with type 2 diabetes

We designed an experiment to examine the effect of bile acid sequestration with Colesevelam on fasting and postprandial glucose metabolism in type 2 diabetes. To do so, we tested the hypothesis that Colesevelam increases the disposition index (DI), and this increase is associated with increased glucagon-like peptide-1 (GLP-1) concentrations. Thirty-eight subjects on metformin monotherapy were studied using a double-blind, placebo-controlled, parallel-group design. Subjects were studied before and after 12 weeks of Colesevelam or placebo using a labeled triple-tracer mixed meal to measure the rate of meal appearance (Meal Ra), endogenous glucose production (EGP), and glucose disappearance (Rd). Insulin sensitivity and β-cell responsivity indices were estimated using the oral minimal model and then used to calculate DI. Therapy with Colesevelam was associated with a decrease in fasting (7.0 ± 0.2 vs. 6.6 ± 0.2 mmol/L; P = 0.004) and postprandial glucose concentrations (3,145 ± 138 vs. 2,896 ± 127 mmol/6 h; P = 0.01) in the absence of a change in insulin concentrations. Minimal model-derived indices of insulin secretion and action were unchanged. Postprandial GLP-1 concentrations were not altered by Colesevelam. Although EGP and Rd were unchanged, integrated Meal Ra was decreased by Colesevelam (5,191 ± 204 vs. 5,817 ± 204 μmol/kg/6 h; P = 0.04), suggesting increased splanchnic sequestration of meal-derived glucose.

Weight loss induced by Roux-en-Y gastric bypass but not laparoscopic adjustable gastric banding increases circulating bile acids

CONTEXT

It has been hypothesized that increased plasma bile acids (BAs) contribute to metabolic improvements after Roux-en-Y gastric bypass (RYGB) surgery by the G protein-coupled receptor TGR5-mediated effects on glucagon-like peptide-1 secretion and thyroid hormones.

OBJECTIVE

The objective of this study was to evaluate the importance of bariatric surgery-induced alterations in BA physiology on factors that regulate glucose homeostasis (insulin secretion and sensitivity) and energy metabolism (resting energy expenditure and thyroid hormone axis). DESIGN, PARTICIPANTS, INTERVENTION, AND MAIN OUTCOME MEASURE: Eighteen extremely obese subjects were studied before and after 20% weight loss, induced by either laparoscopic adjustable gastric banding (LAGB) (n = 10) or RYGB surgery (n = 8).

RESULTS

Plasma BAs more than doubled after RYGB [fasting: 1.08 (0.26-1.42) to 2.28 (1.59-3.28) μmol/L, P = .03; postprandial: 2.46 ± 1.59 to 6.00 ± 2.75 μmol/L, P = .01] but were either lower or did not change after LAGB [fasting: 1.80 (1.49-2.19) to 0.92 (0.73-1.15) μmol/L, P = .02; postprandial: 3.71 ± 2.61 to 2.82 ± 1.75 μmol/L, P = .14]. Skeletal muscle expression of TGR5 targets, Kir6.2 and cyclooxygenase IV, increased after RYGB but not LAGB. Surgery-induced changes in BAs were associated with increased peak postprandial plasma glucagon-like peptide-1 (r(2) = 0.509, P = .001) and decreased serum TSH (r(2) = 0.562, P < .001) but did not correlate with the change in insulin response to a meal (r(2) = 0.013, P = .658), insulin sensitivity (assessed as insulin stimulated glucose disposal during a hyperinsulinemic-euglycemic clamp procedure) (r(2) = 0.001, P = .995), or resting energy expenditure (r(2) = 0.004, P = .807).

CONCLUSIONS

Compared with LAGB, RYGB increases circulating BAs and TGR5 signaling, but this increase in BAs is not a significant predictor of changes in glucose homeostasis or energy metabolism.

Effects of taurocholic acid on glycemic, glucagon-like peptide-1, and insulin responses to small intestinal glucose infusion in healthy humans

CONTEXT

In vitro and animal studies suggest that bile acids have the capacity to reduce blood glucose by stimulating glucagon-like peptide-1 (GLP-1) and, thereby, insulin.

OBJECTIVE

This study evaluated the effects of intrajejunal taurocholic acid (TCA) on blood glucose, GLP-1, and insulin responses to jejunal glucose infusion in healthy men.

PARTICIPANTS AND DESIGN

Ten healthy men were each studied on 2 days in a double-blind, randomized order. After the subjects fasted overnight, a jejunal catheter was positioned and a balloon inflated 30 cm beyond the pylorus with aspiration of endogenous bile. Two grams TCA in saline, or saline control, was infused beyond the balloon over 30 minutes, followed by 2 g TCA or control, together with 60 g glucose, over the next 120 minutes. Blood was sampled frequently for the measurements of blood glucose, total GLP-1, insulin, C-peptide, and glucagon.

RESULTS

Intrajejunal infusion of TCA alone (t = -30 to 0 minutes) had no effect on blood glucose, GLP-1, insulin, C-peptide, or glucagon concentrations. During intrajejunal glucose infusion (t = 0 to 120 minutes), blood glucose concentrations were lower (P < .001), and plasma GLP-1 (P < .001) and the C-peptide/glucose ratio (P = .008) were both greater, whereas plasma insulin, C-peptide, and glucagon levels were not significantly different after TCA than after control.

CONCLUSIONS

In healthy humans, small intestinal infusion of TCA potently reduces the glycemic response to small intestinal glucose, associated with an increase in GLP-1 and C-peptide/glucose ratio. These observations indicate the potential for bile acid-based therapy in type 2 diabetes.

Epigenetics of insulin resistance: an emerging field in translational medicine

In this article, we review the current knowledge of and recent insights into the role of epigenetic factors in the development of insulin resistance (IR), with emphasis on peroxisome proliferator-activated receptor gamma coactivator 1α (PPARGC1A or PGC1α) methylation on fetal programming and liver modulation of glucose-related phenotypes. We discuss the pathogenesis of IR beyond the integrity of β-cell function and illustrate the novel concept of mitochondrial epigenetics to explain the pathobiology of metabolic-syndrome-related phenotypes. Moreover, we discuss whether epigenetic marks in genes of the circadian rhythm system are able to modulate insulin/glucose-related metabolic functions and place hypoxia inducible factor 1 α (HIF1α) as a part of the master CLOCK gene/protein interaction network that might modulate IR. Finally, we highlight relevant information about epigenetic marks and IR so that clinicians practicing in the community may envision future areas of medical intervention and predict putative biomarkers for early disease detection.

Role of GLP-1 in the Hypoglycemic Effects of Wild Bitter Gourd

This study aimed to examine the role of GLP-1 in the hypoglycemic activity of wild bitter gourd (Momordica charantia L., BG). In vitro, the GLP-1 secretion in STC-1, a murine enteroendocrine cell line, was dose dependently stimulated by water extract (WE), its fractions (WEL, >3 kD and WES, <3 kD), and a bitter compounds-rich fraction of BG. These stimulations were partially inhibited by probenecid, a bitter taste receptor inhibitor, and by U-73122, a phospholipase Cβ2 inhibitor. These results suggested that the stimulation might involve, at least in part, certain bitter taste receptors and/or PLCβ2-signaling pathway. Two cucurbitane triterpenoids isolated from BG, 19-nor-cucurbita-5(10),6,8,22-(E),24-pentaen-3β-ol, and 5β,19-epoxycucurbita-6,24-diene-3β,23ξ-diol (karavilagenine E,) showed relative high efficacy in the stimulation. In vivo, mice fed BG diet showed higher insulinogenic index in an oral glucose tolerance test. A single oral dose of WE or WES pretreatment significantly improved intraperitoneal glucose tolerance. A single oral dose of WES significantly decreased glucose and increased insulin and GLP-1 in serum after 30 min. This acute hypoglycemic effect of WES was abolished by pretreatment with exendin-9, a GLP-1 receptor antagonist. Our data provide evidence that BG stimulates GLP-1 secretion which contributes, at least in part, to the antidiabetic activity of BG through an incretin effect.

The TGR5 receptor mediates bile acid-induced itch and analgesia

Patients with cholestatic disease exhibit pruritus and analgesia, but the mechanisms underlying these symptoms are unknown. We report that bile acids, which are elevated in the circulation and tissues during cholestasis, cause itch and analgesia by activating the GPCR TGR5. TGR5 was detected in peptidergic neurons of mouse dorsal root ganglia and spinal cord that transmit itch and pain, and in dermal macrophages that contain opioids. Bile acids and a TGR5-selective agonist induced hyperexcitability of dorsal root ganglia neurons and stimulated the release of the itch and analgesia transmitters gastrin-releasing peptide and leucine-enkephalin. Intradermal injection of bile acids and a TGR5-selective agonist stimulated scratching behavior by gastrin-releasing peptide- and opioid-dependent mechanisms in mice. Scratching was attenuated in Tgr5-KO mice but exacerbated in Tgr5-Tg mice (overexpressing mouse TGR5), which exhibited spontaneous pruritus. Intraplantar and intrathecal injection of bile acids caused analgesia to mechanical stimulation of the paw by an opioid-dependent mechanism. Both peripheral and central mechanisms of analgesia were absent from Tgr5-KO mice. Thus, bile acids activate TGR5 on sensory nerves, stimulating the release of neuropeptides in the spinal cord that transmit itch and analgesia. These mechanisms could contribute to pruritus and painless jaundice that occur during cholestatic liver diseases.

The TGR5 gene is expressed in human subcutaneous adipose tissue and is associated with obesity, weight loss and resting metabolic rate

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Human adipose tissue (AT) expresses the bile acid receptor TGR5.

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Human AT TGR5 expression is linked to obesity.

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Resting metabolic rate and AT TGR5 expression is positively correlated.

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TGR5 expression is not higher in brown compared to white human AT.

Bile acids have emerged as a new class of signaling molecules that play a role in metabolism. Studies in mice have shown that the bile acid receptor TGR5 mediates several of these effects but the metabolic function of TGR5 in humans is less well established. Here we show that human adipose tissue TGR5 expression is positively correlated to obesity and reduced during diet-induced weight loss. Adipose tissue TGR5 expression was also positively correlated to resting metabolic rate. Our study indicates that human adipose tissue contributes to the TGR5 mediated metabolic effects of bile acids and plays a role in energy expenditure.

Hepatic glucose sensing is required to preserve β cell glucose competence

Liver glucose metabolism plays a central role in glucose homeostasis and may also regulate feeding and energy expenditure. Here we assessed the impact of glucose transporter 2 (Glut2) gene inactivation in adult mouse liver (LG2KO mice). Loss of Glut2 suppressed hepatic glucose uptake but not glucose output. In the fasted state, expression of carbohydrate-responsive element-binding protein (ChREBP) and its glycolytic and lipogenic target genes was abnormally elevated. Feeding, energy expenditure, and insulin sensitivity were identical in LG2KO and control mice. Glucose tolerance was initially normal after Glut2 inactivation, but LG2KO mice exhibited progressive impairment of glucose-stimulated insulin secretion even though β cell mass and insulin content remained normal. Liver transcript profiling revealed a coordinated downregulation of cholesterol biosynthesis genes in LG2KO mice that was associated with reduced hepatic cholesterol in fasted mice and reduced bile acids (BAs) in feces, with a similar trend in plasma. We showed that chronic BAs or farnesoid X receptor (FXR) agonist treatment of primary islets increases glucose-stimulated insulin secretion, an effect not seen in islets from Fxr(-/-) mice. Collectively, our data show that glucose sensing by the liver controls β cell glucose competence and suggest BAs as a potential mechanistic link.

Gustducin couples fatty acid receptors to GLP-1 release in colon

Sweet taste receptor subunits and α-gustducin found in enteroendocrine cells of the small intestine have been implicated in release of the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) in response to glucose and noncaloric sweeteners. α-Gustducin has also been found in colon, although its function there is unclear. We examined expression of α-gustducin, GLP-1, and GIP throughout the intestine. The number of α-gustducin-expressing cells and those coexpressing α-gustducin together with GLP-1 and/or GIP increased from small intestine to colon. α-Gustducin also was coexpressed with fatty acid G protein-coupled receptor (GPR) 40, GPR41, GPR43, GPR119, GPR120, and bile acid G protein-coupled receptor TGR5 in enteroendocrine cells of the colon. In colon, GPR43 was coexpressed with GPR119 and GPR120, but not with TGR5. Treatment of colonic mucosa isolated from wild-type mice with acetate, butyrate, oleic acid, oleoylethanolamide, or lithocholic acid stimulated GLP-1 secretion. However, GLP-1 release in response to these fatty acids was impaired in colonic tissue from α-gustducin knockout mice.

Bile acid receptor activation modulates hepatic monocyte activity and improves nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) affects a large proportion of the American population. The spectrum of disease ranges from bland steatosis without inflammation to nonalcoholic steatohepatitis and cirrhosis. Bile acids are critical regulators of hepatic lipid and glucose metabolism and signal through two major receptor pathways: farnesoid X receptor (FXR), a member of the nuclear hormone receptor superfamily, and TGR5, a G protein-coupled bile acid receptor (GPBAR1). Both FXR and TGR5 demonstrate pleiotropic functions, including immune modulation. To evaluate the effects of these pathways in NAFLD, we treated obese db/db mice with a dual FXR/TGR5 agonist (INT-767) for 6 weeks. Treatment with the agonist significantly improved the histological features of nonalcoholic steatohepatitis. Furthermore, treatment increased the proportion of intrahepatic monocytes with the anti-inflammatory Ly6C(low) phenotype and increased intrahepatic expression of genes expressed by alternatively activated macrophages, including CD206, Retnla, and Clec7a. In vitro treatment of monocytes with INT-767 led to decreased Ly6C expression and increased IL-10 production through a cAMP-dependent pathway. Our data indicate that FXR/TGR5 activation coordinates the immune phenotype of monocytes and macrophages, both in vitro and in vivo, identifying potential targeting strategies for treatment of NAFLD.

Activation of G protein-coupled bile acid receptor, TGR5, induces smooth muscle relaxation via both Epac- and PKA-mediated inhibition of RhoA/Rho kinase pathway

The present study characterized the TGR5 expression and the signaling pathways coupled to this receptor that mediates the relaxation of gastric smooth muscle. TGR5 was detected in gastric muscle cells by RT-PCR and Western blotting. Treatment of cells with the TGR5-selective ligand oleanolic acid (OA) activated Gαs, but not Gαq, Gαi1, Gαi2, or Gαi3, and increased cAMP levels. OA did not elicit contraction, but caused relaxation of carbachol-induced contraction of gastric muscle cells from wild-type mice, but not tgr5(-/-) mice. OA, but not a selective exchange protein activated by cAMP (Epac) ligand (8-pCPT-2'-O-Me-cAMP), caused phosphorylation of RhoA and the phosphorylation was blocked by the PKA inhibitor, myristoylated PKI, and by the expression of phosphorylation-deficient mutant RhoA (S188A). Both OA and Epac ligand stimulated Ras-related protein 1 (Rap1) and inhibited carbachol (CCh)-induced Rho kinase activity. Expression of RhoA (S188A) or PKI partly reversed the inhibition of Rho kinase activity by OA but had no effect on inhibition by Epac ligand. However, suppression of Rap1 with siRNA blocked the inhibition of Rho kinase by Epac ligand, and partly reversed the inhibition by OA; the residual inhibition was blocked by PKI. Muscle relaxation in response to OA, but not Epac ligand, was partly reversed by PKI. We conclude that activation of TGR5 causes relaxation of gastric smooth muscle and the relaxation is mediated through inhibition of RhoA/Rho kinase pathway via both cAMP/Epac-dependent stimulation of Rap1 and cAMP/PKA-dependent phosphorylation of RhoA at Ser(188). TGR5 receptor activation on smooth muscle reveals a novel mechanism for the regulation of gut motility by bile acids.

Postprandial gut hormone responses and glucose metabolism in cholecystectomized patients

Preclinical studies suggest that gallbladder emptying, via bile acid-induced activation of the G protein-coupled receptor TGR5 in intestinal L cells, may play a significant role in the secretion of the incretin hormone glucagon-like peptide-1 (GLP-1) and, hence, postprandial glucose homeostasis. We examined the secretion of gut hormones in cholecystectomized subjects to test the hypothesis that gallbladder emptying potentiates postprandial release of GLP-1. Ten cholecystectomized subjects and 10 healthy, age-, gender-, and body mass index-matched control subjects received a standardized fat-rich liquid meal (2,200 kJ). Basal and postprandial plasma concentrations of glucose, insulin, C-peptide, glucagon, GLP-1, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-2 (GLP-2), cholecystokinin (CCK), and gastrin were measured. Furthermore, gastric emptying and duodenal and serum bile acids were measured. We found similar basal glucose concentrations in the two groups, whereas cholecystectomized subjects had elevated postprandial glucose excursions. Cholecystectomized subjects had reduced postprandial concentrations of duodenal bile acids, but preserved postprandial plasma GLP-1 responses, compared with control subjects. Also, cholecystectomized patients exhibited augmented fasting glucagon. Basal plasma CCK concentrations were lower and peak concentrations were higher in cholecystectomized patients. The concentrations of GIP, GLP-2, and gastrin were similar in the two groups. In conclusion, cholecystectomized subjects had preserved postprandial GLP-1 responses in spite of decreased duodenal bile delivery, suggesting that gallbladder emptying is not a prerequisite for GLP-1 release. Cholecystectomized patients demonstrated a slight deterioration of postprandial glycemic control, probably because of metabolic changes unrelated to incretin secretion.

Colesevelam suppresses hepatic glycogenolysis by TGR5-mediated induction of GLP-1 action in DIO mice

Bile acid sequestrants are nonabsorbable resins designed to treat hypercholesterolemia by preventing ileal uptake of bile acids, thus increasing catabolism of cholesterol into bile acids. However, sequestrants also improve hyperglycemia and hyperinsulinemia through less characterized metabolic and molecular mechanisms. Here, we demonstrate that the bile acid sequestrant, colesevelam, significantly reduced hepatic glucose production by suppressing hepatic glycogenolysis in diet-induced obese mice and that this was partially mediated by activation of the G protein-coupled bile acid receptor TGR5 and glucagon-like peptide-1 (GLP-1) release. A GLP-1 receptor antagonist blocked suppression of hepatic glycogenolysis and blunted but did not eliminate the effect of colesevelam on glycemia. The ability of colesevelam to induce GLP-1, lower glycemia, and spare hepatic glycogen content was compromised in mice lacking TGR5. In vitro assays revealed that bile acid activation of TGR5 initiates a prolonged cAMP signaling cascade and that this signaling was maintained even when the bile acid was complexed to colesevelam. Intestinal TGR5 was most abundantly expressed in the colon, and rectal administration of a colesevelam/bile acid complex was sufficient to induce portal GLP-1 concentration but did not activate the nuclear bile acid receptor farnesoid X receptor (FXR). The beneficial effects of colesevelam on cholesterol metabolism were mediated by FXR and were independent of TGR5/GLP-1. We conclude that colesevelam administration functions through a dual mechanism, which includes TGR5/GLP-1-dependent suppression of hepatic glycogenolysis and FXR-dependent cholesterol reduction.

Obesity surgery: happy with less or eternally hungry?

The superior efficacy of bariatric surgery compared with intensive medical treatment in reversing metabolic disease is now well accepted, but the critical mechanisms remain unknown. Unlike dieting, which triggers strong counter-regulatory responses such as hunger and craving, some obesity surgeries appear to permanently reset the level of defended body weight. Understanding the molecular mechanisms behind successful surgery would thus go a long way in developing future 'knifeless' treatment options. Major candidates include changes in gut-brain signaling by hormones, bile acids, and other still unidentified factors. By re-sensitizing homeostatic regulatory circuits in the hypothalamus and hedonic-motivational processing in corticolimbic systems to internal signals, bariatric surgery could thus lead to a state of being content with less.

Deficiency of G-protein-coupled bile acid receptor Gpbar1 (TGR5) enhances chemically induced liver carcinogenesis

Gpbar1 (TGR5), a membrane-bound bile acid receptor, is well known for its roles in regulation of energy homeostasis and glucose metabolism. TGR5 activation also inhibits nuclear factor κB (NF-κB)-mediated inflammation. Here we show that TGR5 deficiency enhances chemically induced liver carcinogenesis, and that TGR5 is a negative regulator of signal transducer and activator of transcription 3 (STAT3) signaling. Mice lacking TGR5 were much more susceptible to diethylnitrosamine (DEN)-induced acute liver injury and liver carcinogenesis than wildtype (WT) mice. Consistent with the increasing incidence of liver cancer in TGR5(-/-) mice, hepatocyte death, compensatory proliferation, and gene expression of certain inflammatory cytokines and matrix metalloproteinases were more sensitive to DEN induction in the absence of TGR5 signaling. In vitro, TGR5 activation greatly inhibited proliferation and migration of human liver cancer cells. We then found that TGR5 activation strongly suppressed STAT3 signaling in vitro and in vivo. Furthermore, we observed that TGR5 antagonizes the STAT3 pathway through suppressing STAT3 phosphorylation, its transcription activity, and DNA binding activity, which suggests that TGR5 antagonizes liver tumorigenesis at least in part by inhibiting STAT3 signaling.

CONCLUSION

These findings identify TGR5 as a novel liver tumor suppressor that may serve as an attractive therapeutic tool for human liver cancer.

Digestive physiology of the pig symposium: involvement of gut chemosensing in the regulation of mucosal barrier function and defense mechanisms

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.

Incretin secretion stimulated by ursodeoxycholic acid in healthy subjects

Bile acids play an important role in post-prandial glucose metabolism by stimulating release of glucagon-like peptide-1 (GLP-1) via the G-protein-coupled receptor TGR5, which is expressed in intestinal L cells. Thus, bile acid sequestrants are expected to stimulate secretion of endogenous GLP-1 through TGR5. We investigated incretin and insulin secretion after a meal with and without ursodeoxycholic acid (UDCA), a widely used therapeutic agent in liver diseases, in 7 non-diabetic Japanese subjects. We found that UDCA intake resulted in higher GLP-1 secretion (area under the curve [AUC] of 0–60 min after meal without UDCA, 450 ± 162 mmol·min/l; with UDCA, 649 ± 232 mmol·min/l, P = 0.046) and lower blood glucose (AUC of 0–60 min without UDCA, 7191 ± 250 mg·min/dl; with UDCA, 6716 ± 189 mg·min/dl, P = 0.001) , although we did not find statistically significant insulin increase by UDCA intake (AUC of 0–60 min without UDCA, 1551 ± 418 μU·min/ml; with UDCA, 1941 ± 246 μU·min/ml, P = 0.065). These results suggest that UDCA increases bile-induced GLP-1 secretion. Ours is the first report showing increased GLP-1 secretion and decreased blood glucose in response to UDCA.

Identification of Tetrahydropyrido[4,3-d]pyrimidine Amides as a New Class of Orally Bioavailable TGR5 Agonists

Takeda G-protein-coupled receptor 5 (TGR5) represents an exciting biological target for the potential treatment of diabetes and metabolic syndrome. A new class of high-throughput screening (HTS)-derived tetrahydropyrido[4,3-d]pyrimidine amide TGR5 agonists is disclosed. We describe our effort to identify an orally available agonist suitable for assessment of systemic TGR5 agonism. This effort resulted in identification of 16, which had acceptable potency and pharmacokinetic properties to allow for in vivo assessment in dog. A key aspect of this work was the calibration of human and dog in vitro assay systems that could be linked with data from a human ex vivo peripheral blood monocyte assay that expresses receptor at endogenous levels. Potency from the human in vitro assay was also found to correlate with data from an ex vivo human whole blood assay. This calibration exercise provided confidence that 16 could be used to drive plasma exposures sufficient to test the effects of systemic activation of TGR5.

Conjugated bile acids associate with altered rates of glucose and lipid oxidation after Roux-en-Y gastric bypass

BACKGROUND

Laparoscopic Roux-en-Y gastric bypass (RYGB) induces a more favorable metabolic profile than expected by weight loss alone. In this study, we investigated the effect of RYGB on serum bile acid levels and their relation to clinical outcomes.

METHODS

We included 30 obese patients who underwent RYGB (BMI = 46.1 ± 5.9 kg/m(2)). Clinical measurements and laboratory determinations were performed before surgery and 1 year after surgery. Fasting serum bile acids were measured by an enzymatic method and individual bile acids were quantified by HLPC-tandem mass spectrometry. Indirect calorimetry was performed to measure the rates of energy expenditure and substrate oxidation.

RESULTS

Fasting total serum bile acid levels increased twofold after RYGB (pre, 3.68 ± 2.03 vs. post, 7.06 ± 9.65 μmol/l, +92 %, p = 0.002). This increase in total bile acids was accompanied by a decrease in conjugated bile acids, which correlated with decreased glucose oxidation (r = 0.571, p = 0.002) and with increased lipid oxidation (r = -0.626, p = 0.0004). The change in taurine-conjugated bile acids correlated with altered DIO2 mRNA expression in adipose tissue (r = -0.498, p = 0.013) potentially linking bile acid conjugation to substrate oxidation through DIO2.

CONCLUSIONS

Fasting serum bile acid levels increase after RYGB. More specifically, changes in bile acid conjugation after RYGB associate with altered energy metabolism.

Bile acid transporters and regulatory nuclear receptors in the liver and beyond

Bile acid (BA) transporters are critical for maintenance of the enterohepatic BA circulation where BAs exert their multiple physiological functions including stimulation of bile flow, intestinal absorption of lipophilic nutrients, solubilization and excretion of cholesterol, as well as antimicrobial and metabolic effects. Tight regulation of BA transporters via nuclear receptors is necessary to maintain proper BA homeostasis. Hereditary and acquired defects of BA transporters are involved in the pathogenesis of several hepatobiliary disorders including cholestasis, gallstones, fatty liver disease and liver cancer, but also play a role in intestinal and metabolic disorders beyond the liver. Thus, pharmacological modification of BA transporters and their regulatory nuclear receptors opens novel treatment strategies for a wide range of disorders.

Dipeptidyl peptidase 4-deficient rats have improved bile secretory function in high fat diet-induced steatosis

BACKGROUND/AIMS

Rodent obesity models have been shown to display impaired bile secretory functions. We have shown that glucagon-like peptide 1 (GLP-1) attenuates hepatic lipogenesis, and in the present study we investigated whether GLP-1 also improves high fat diet-associated cholestatic injury.

METHODS

Wild type (WT) and dipeptidyl peptidase 4-deficient rats (DPP4-) with chronic elevated serum levels of active GLP-1 were fed regular chow and a Western diet for 2 months. Primary hepatocytes were used to assess GLP-1 effects on mRNA expression and transcription of genes encoding bile acid synthesis enzymes and transporters.

RESULTS

DPP4- exhibited attenuated liver injury as expressed by lower serum AST and ALT after 2 months of a Western diet. In addition, DPP4- had better insulin sensitivity, lower serum triglycerides, cholesterol and bile acids. Hepatic expression of cyp7A1, the rate limiting enzyme in conversion of cholesterol into bile acids, was strongly attenuated in DPP4- fed with a Western diet. Moreover, hepatic expression of bile transporter, ABCB11, was increased, facilitating a higher rate of bile secretion. Mechanistically, we showed that GLP-1 directly reduced basal and LXR-induced cyp7A1 mRNA expression and suppressed cyp7A1 transcription in transient transfection assays in primary hepatocytes. However, GLP-1 and its analog exendin 4 also induced mRNA expression of bile acid transporter ABCC3 in primary rat hepatocyte cultures.

CONCLUSIONS

Our data suggest that GLP-1 analogs may serve as a novel therapeutic drug to alleviate obesity-induced liver injury by reducing bile acid synthesis and improving liver bile secretory function.

Role of nuclear receptors in lipid dysfunction and obesity-related diseases

This article is a report on a symposium sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the Experimental Biology 12 meeting in San Diego, CA. The presentations discussed the roles of a number of nuclear receptors in regulating glucose and lipid homeostasis, the pathophysiology of obesity-related disease states, and the promise associated with targeting their activities to treat these diseases. While many of these receptors (in particular, constitutive androstane receptor and pregnane X receptor) and their target enzymes have been thought of as regulators of drug and xenobiotic metabolism, this symposium highlighted the advances made in our understanding of the endogenous functions of these receptors. Similarly, as we gain a better understanding of the mechanisms underlying bile acid signaling pathways in the regulation of body weight and glucose homeostasis, we see the importance of using complementary approaches to elucidate this fascinating network of pathways. The observation that some receptors, like the farnesoid X receptor, can function in a tissue-specific manner via well defined mechanisms has important clinical implications, particularly in the treatment of liver diseases. Finally, the novel findings that agents that selectively activate estrogen receptor β can effectively inhibit weight gain in a high-fat diet model of obesity identifies a new role for this member of the steroid superfamily. Taken together, the significant findings reported during this symposium illustrate the promise associated with targeting a number of nuclear receptors for the development of new therapies to treat obesity and other metabolic disorders.

TRC210258, a novel TGR5 agonist, reduces glycemic and dyslipidemic cardiovascular risk in animal models of diabesity

BACKGROUND

Patients with diabesity have a significantly increased risk of developing cardiovascular disease. Therefore, therapy addressing the multiple metabolic abnormalities linked with diabesity and leading to further reduction of cardiovascular risk is highly desirable. Activation of the TGR5 receptor holds therapeutic potential for diabesity. In the present study, we evaluated the efficacy of TRC210258, a novel TGR5 agonist, in clinically relevant animal models of diabesity.

METHODS

A novel small molecule, TRC210258 (N-(4-chlorophenyl)-2-(4-fluorophenoxy)-N-methylimidazo (1, 2-a) pyrimidine-3-carboxamide), was synthesized. The in vitro TGR5 receptor activation potential of TRC210258 was assessed by cyclic adenosine monophosphate (cAMP) assay and cAMP-responsive element reporter assay using cells overexpressing the human TGR5 receptor. The effect of TRC210258 on glucagon-like peptide-1 release was evaluated in vitro using a human enteroendocrine cell line. The effect of TRC210258 on energy expenditure and glycemic control was evaluated in high-fat diet-induced obese mice. Additionally, the effect of TRC210258 on dyslipidemic parameters was determined in high fat-fed hamsters.

RESULTS

TRC210258 demonstrated potent TGR5 agonist activity, with enhanced glucagon-like peptide-1 release and energy expenditure. Treatment with TRC210258 resulted in better glycemic control and improved parameters of dyslipidemia such as plasma triglyceride, low-density lipoprotein cholesterol, and non-high-density lipoprotein cholesterol levels. Treatment with TRC210258 also improved emerging dyslipidemic cardiovascular risk parameters, including remnant cholesterol and triglyceride clearance.

CONCLUSION

This study highlights the potential of TRC210258, a novel TGR5 agonist, to improve dyslipidemic cardiovascular risk beyond glycemic control in patients with type 2 diabetes.

Optimization of triazole-based TGR5 agonists towards orally available agents

With the challenge of striking the balance of TGR5 potency and clearance, the screening strategy as well as medicinal chemistry strategy are discussed in this article.

FXR activation normalizes insulin sensitivity in visceral preadipocytes of a rabbit model of MetS

Insulin resistance is the putative key underlying mechanism linking adipose tissue (AT) dysfunction with liver inflammation and steatosis in metabolic syndrome (MetS). We have recently demonstrated that the selective farnesoid X receptor (FXR) agonist obeticholic acid (OCA) ameliorates insulin resistance and the metabolic profile with a marked reduction in the amount of visceral AT (VAT) in a high-fat diet (HFD)-induced rabbit model of MetS. These effects were mediated by the activation of FXR, since treatment with the selective TGR5 agonist INT-777 was not able to ameliorate the metabolic parameters evaluated. Herein, we report the effects of in vivo OCA dosing on the liver, the VAT, and the adipogenic capacity of VAT preadipocytes (rPADs) isolated from rabbits on a HFD compared with those on a control diet. VAT and liver were studied by immunohistochemistry, Western blot analysis, and RT-PCR. rPADs were exposed to a differentiating mixture to evaluate adipogenesis. Adipocyte size, hypoxia, and the expression of perilipin and cytosolic insulin-regulated glucose transporter GLUT4 (SLC2A4) were significantly increased in VAT isolated from the HFD rabbits, and normalized by OCA. The expression of steatosis and inflammation markers was increased in the liver of the HFD rabbits and normalized by OCA. rPADs isolated from the HFD rabbits were less sensitive to insulin, as demonstrated by the decreased insulin-induced glucose uptake, triglyceride synthesis, and adipogenic capacity, as well as by the impaired fusion of lipid droplets. OCA treatment preserved all the aforementioned metabolic functions. In conclusion, OCA dosing in a MetS rabbit model ameliorates liver and VAT functions. This could reflect the ability of OCA to restore insulin sensitivity in AT unable to finalize its storage function, counteracting MetS-induced metabolic alterations and pathological AT deposition.

Developments in understanding bile acid metabolism

Bile acids (BAs) are not only facilitators participating in the absorption of dietary lipids and soluble vitamins, but are also important signaling molecules exerting versatile biophysiological effects. Three major signaling pathways, including the MAPK pathways, the nuclear hormone receptor farnesoid X receptor a-mediated pathways and the G protein-coupled receptor TGR5/M-BAR-mediated pathways, have been identified to be the targets of BAs. BAs, the biologically many-sided and toxic molecules, regulate the homeostasis of themselves via these signaling pathways. BAs also affect diverse metabolic status including glucose metabolism, lipid metabolism, energy expenditure, immunity and others. BAs and their related signaling mechanisms are attractive therapeutic targets of various diseases such as metabolic syndrome.

The receptor TGR5 mediates the prokinetic actions of intestinal bile acids and is required for normal defecation in mice

BACKGROUND & AIMS

Abnormal delivery of bile acids (BAs) to the colon as a result of disease or therapy causes constipation or diarrhea by unknown mechanisms. The G protein-coupled BA receptor TGR5 (or GPBAR1) is expressed by enteric neurons and endocrine cells, which regulate motility and secretion.

METHODS

We analyzed gastrointestinal and colon transit, as well as defecation frequency and water content, in wild-type, knockout, and transgenic mice (trg5-wt, tgr5-ko, and tgr5-tg, respectively). We analyzed colon tissues for contractility, peristalsis, and transmitter release.

RESULTS

Deoxycholic acid inhibited contractility of colonic longitudinal muscle from tgr5-wt but not tgr5-ko mice. Application of deoxycholic acid, lithocholic acid, or oleanolic acid (a selective agonist of TGR5) to the mucosa of tgr5-wt mice caused oral contraction and caudal relaxation, indicating peristalsis. BAs stimulated release of the peristaltic transmitters 5-hydroxytryptamine and calcitonin gene-related peptide; antagonists of these transmitters suppressed BA-induced peristalsis, consistent with localization of TGR5 to enterochromaffin cells and intrinsic primary afferent neurons. tgr5-ko mice did not undergo peristalsis or transmitter release in response to BAs. Mechanically induced peristalsis and transmitter release were not affected by deletion of tgr5. Whole-gut transit was 1.4-fold slower in tgr5-ko than tgr5-wt or tgr5-tg mice, whereas colonic transit was 2.2-fold faster in tgr5-tg mice. Defecation frequency was reduced 2.6-fold in tgr5-ko and increased 1.4-fold in tgr5-tg mice compared with tgr5-wt mice. Water content in stool was lower (37%) in tgr5-ko than tgr5-tg (58%) or tgr5-wt mice (62%).

CONCLUSIONS

The receptor TGR5 mediates the effects of BAs on colonic motility, and deficiency of TGR5 causes constipation in mice. These findings might mediate the long-known laxative properties of BAs, and TGR5 might be a therapeutic target for digestive diseases.

Bile acids in the colon, from healthy to cytotoxic molecules

Bile acids are natural detergents mainly involved in facilitating the absorption of dietary fat in the intestine. In addition to this absorptive function, bile acids are also essential in the maintenance of the intestinal epithelium homeostasis. To accomplish this regulatory function, bile acids may induce programmed cell death fostering the renewal of the epithelium. Here we first discuss on the different molecular pathways of cell death focusing on apoptosis in colon epithelial cells. Bile acids may induce apoptosis in colonocytes through different mechanisms. In contrast to hepatocytes, the extrinsic apoptotic pathway seems to have a low relevance regarding bile acid cytotoxicity in the colon. On the contrary, these molecules mainly trigger apoptosis through direct or indirect mitochondrial perturbations, where oxidative stress plays a key role. In addition, bile acids may also act as regulatory molecules involved in different cell signaling pathways in colon cells. On the other hand, there is increasing evidence that the continuous exposure to certain hydrophobic bile acids, due to a fat-rich diet or pathological conditions, may induce oxidative DNA damage that, in turn, may lead to colorectal carcinogenesis as a consequence of the appearance of cell populations resistant to bile acid-induced apoptosis. Finally, some bile acids, such as UDCA, or low concentrations of hydrophobic bile acids, can protect colon cells against apoptosis induced by high concentrations of cytotoxic bile acids, suggesting a dual behavior of these agents as pro-death or pro-survival molecules.

Bile Acid Receptors and Liver Cancer

Liver cancer, particularly hepatocellular carcinoma (HCC), is the third leading cause of cancer death in the world. Bile acids (BAs) are liver-produced amphipathic molecules that are required to facilitate the absorption of cholesterol, fat-soluble vitamins, and lipids in the intestine. However, BAs are also known to act as potential carcinogens and deregulation of BA homeostasis has been linked to HCC formation. Two key BA receptors, farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (TGR5), were recently identified, which provides great insights into BAs' normal physiological functions as well as their carcinogenic effects. In this review, we focus on the potential links among BAs, two BA receptors, and HCC. FXR and TGR5 not only play key roles in regulating BA homeostasis but also are essential in suppressing BAs' carcinogenic effects on liver cancer.

Bile-acid-mediated decrease in endoplasmic reticulum stress: a potential contributor to the metabolic benefits of ileal interposition surgery in UCD-T2DM rats

Post-operative increases in circulating bile acids have been suggested to contribute to the metabolic benefits of bariatric surgery; however, their mechanistic contributions remain undefined. We have previously reported that ileal interposition (IT) surgery delays the onset of type 2 diabetes in UCD-T2DM rats and increases circulating bile acids, independently of effects on energy intake or body weight. Therefore, we investigated potential mechanisms by which post-operative increases in circulating bile acids improve glucose homeostasis after IT surgery. IT, sham or no surgery was performed on 2-month-old weight-matched male UCD-T2DM rats. Animals underwent an oral fat tolerance test (OFTT) and serial oral glucose tolerance tests (OGTT). Tissues were collected at 1.5 and 4.5 months after surgery. Cell culture models were used to investigate interactions between bile acids and ER stress. IT-operated animals exhibited marked improvements in glucose and lipid metabolism, with concurrent increases in postprandial glucagon-like peptide-1 (GLP-1) secretion during the OFTT and OGTTs, independently of food intake and body weight. Measurement of circulating bile acid profiles revealed increases in circulating total bile acids in IT-operated animals, with a preferential increase in circulating cholic acid concentrations. Gut microbial populations were assessed as potential contributors to the increases in circulating bile acid concentrations, which revealed proportional increases in Gammaproteobacteria in IT-operated animals. Furthermore, IT surgery decreased all three sub-arms of ER stress signaling in liver, adipose and pancreas tissues. Amelioration of ER stress coincided with improved insulin signaling and preservation of β-cell mass in IT-operated animals. Incubation of hepatocyte, adipocyte and β-cell lines with cholic acid decreased ER stress. These results suggest that postoperative increases in circulating cholic acid concentration contribute to improvements in glucose homeostasis after IT surgery by ameliorating ER stress.

Endogenous metabolites as ligands for G protein-coupled receptors modulating risk factors for metabolic and cardiovascular disease

During the last decade, several G protein-coupled receptors activated by endogenous metabolites have been described. These receptors respond to fatty acids, mono- and disaccharides, amino acids, or various intermediates and products of metabolism, including ketone bodies, lactate, succinate, or bile acids. Receptors of endogenous metabolites are expressed in taste cells, the gastrointestinal tract, adipose tissue, endocrine glands, immune cells, or the kidney and are therefore in a position to sense food intake in the gastrointestinal tract or to link metabolite levels to the appropriate responses of metabolic organs. Some of the receptors appear to provide a link between metabolic and neuronal or immune functions. Given that many of these metabolic processes are dysregulated under pathological conditions, including diabetes, dyslipidemia, and obesity, receptors of endogenous metabolites have also been recognized as potential drug targets to prevent and/or treat metabolic and cardiovascular diseases. This review describes G protein-coupled receptors activated by endogenous metabolites and summarizes their physiological, pathophysiological, and potential pharmacological roles.

Elobixibat for the treatment of constipation

INTRODUCTION

Elobixibat (formerly A3309) is a first-in-class ileal bile acid transporter (IBAT) inhibitor for treatment of chronic idiopathic constipation (CIC; syn functional constipation). CIC affects up to 25% of the general population; and up to a half are unsatisfied with current therapies. There is an unmet need for safe and effective drugs to treat CIC.

AREAS COVERED

The authors present: i) an overview of Phase II clinical trials of elobixibat in CIC, based on peer-reviewed literature and congress presentations and ii) an evaluation of the efficacy and mechanism of action of elobixibat in the treatment of CIC.

EXPERT OPINION

Elobixibat provides a novel approach to treat chronic constipation via IBAT inhibition with enhanced delivery of bile acids to the colon. Pharmacodynamic studies show that it accelerates colonic transit, increases stool frequency, loosens stool consistency and relieves constipation-related symptoms in CIC patients. These beneficial effects are maintained for a minimum of 8 consecutive weeks of treatment. With minimal absorption and low systemic bioavailability, elobixibat is generally well tolerated and may offer the added benefit of improving serum lipid profiles through bile acid depletion.

The bile salt export pump (BSEP) in health and disease

The bile salt export pump (BSEP) is the major transporter for the secretion of bile acids from hepatocytes into bile in humans. Mutations of BSEP are associated with cholestatic liver diseases of varying severity including progressive familial intrahepatic cholestasis type 2 (PFIC-2), benign recurrent intrahepatic cholestasis type 2 (BRIC-2) and genetic polymorphisms are linked to intrahepatic cholestasis of pregnancy (ICP) and drug-induced liver injury (DILI). Detailed analysis of these diseases has considerably increased our knowledge about physiology and pathophysiology of bile secretion in humans. This review focuses on expression, localization, and function, short- and long-term regulation of BSEP as well as diseases association and treatment options for BSEP-associated diseases.

Ghrelin, the proglucagon-derived peptides and peptide YY in nutrient homeostasis

Dysregulation of nutrient homeostasis is implicated in the current epidemics of obesity and type 2 diabetes mellitus. The maintenance of homeostasis in the setting of repeated cycles of feeding and fasting occurs through complex interactions between metabolic, hormonal and neural factors. Although pancreatic islets, the liver, muscle, adipocytes and the central nervous system are all key players in this network, the gastrointestinal tract is the first tissue exposed to ingested nutrients and thus has an important role. This Review focuses on several of the endocrine hormones released by the gastrointestinal tract prior to or during nutrient ingestion that have key roles in maintaining energy balance. These hormones include the gastric orexigenic hormone, ghrelin, and the distal L cell anorexigenic and metabolic hormones, glucagon-like peptide (GLP)-1, GLP-2, oxyntomodulin and peptide YY. Each of these hormones exerts a distinct set of biological actions to maintain nutrient homeostasis, the properties of which are currently, or might soon be, exploited in the clinic for the treatment of obesity and type 2 diabetes mellitus.

Pharmacological activation of the bile acid nuclear farnesoid X receptor is feasible in patients with quiescent Crohn's colitis

Background

The bile acid-activated nuclear receptor Farnesoid X Receptor (FXR) is critical in maintaining intestinal barrier integrity and preventing bacterial overgrowth. Patients with Crohn's colitis (CC) exhibit reduced ileal FXR target gene expression. FXR agonists have been shown to ameliorate inflammation in murine colitis models. We here explore the feasibility of pharmacological FXR activation in CC.

Methods

Nine patients with quiescent CC and 12 disease controls were treated with the FXR ligand chenodeoxycholic acid (CDCA; 15 mg/kg/day) for 8 days. Ileal FXR activation was assessed in the fasting state during 6 hrs after the first CDCA dose and on day 8, by quantification of serum levels of fibroblast growth factor (FGF) 19. Since FGF19 induces gallbladder (GB) refilling in murine models, we also determined concurrent GB volumes by ultrasound. On day 8 ileal and cecal biopsies were obtained and FXR target gene expression was determined.

Results

At baseline, FGF19 levels were not different between CC and disease controls. After the first CDCA dose, there were progressive increases of FGF19 levels and GB volumes during the next 6 hours in CC patients and disease controls (FGF19: 576 resp. 537% of basal; GB volumes: 190 resp. 178% of basal) without differences between both groups, and a further increase at day 8. In comparison with a separate untreated control group, CDCA affected FXR target gene expression in both CC and disease controls, without differences between both groups.

Conclusions

Pharmacological activation of FXR is feasible in patients with CC. These data provide a rationale to explore the anti-inflammatory properties of pharmacological activation of FXR in these patients.

Trial Registration

TrialRegister.nl NTR2009