Intestinal Stearoyl-CoA Desaturase-1 Regulates Energy Balance via Alterations in Bile Acid Homeostasis

BACKGROUND & AIMS

Stearoyl-CoA desaturase-1 (SCD1) converts saturated fatty acids into monounsaturated fatty acids and plays an important regulatory role in lipid metabolism. Previous studies have demonstrated that mice deficient in SCD1 are protected from diet-induced obesity and hepatic steatosis due to altered lipid assimilation and increased energy expenditure. Previous studies in our lab have shown that intestinal SCD1 modulates intestinal and plasma lipids and alters cholesterol metabolism. Here, we investigated a novel role for intestinal SCD1 in the regulation of systemic energy balance.

METHODS

To interrogate the role of intestinal SCD1 in modulating whole body metabolism, intestine-specific Scd1 knockout (iKO) mice were maintained on standard chow diet or challenged with a high-fat diet (HFD). Studies included analyses of bile acid content and composition, and metabolic phenotyping, including body composition, indirect calorimetry, glucose tolerance analyses, quantification of the composition of the gut microbiome, and assessment of bile acid signaling pathways.

RESULTS

iKO mice displayed elevated plasma and hepatic bile acid content and decreased fecal bile acid excretion, associated with increased expression of the ileal bile acid uptake transporter, Asbt. In addition, the alpha and beta diversity of the gut microbiome was reduced in iKO mice, with several alterations in microbe species being associated with the observed increases in plasma bile acids. These increases in plasma bile acids were associated with increased expression of TGR5 targets, including Dio2 in brown adipose tissue and elevated plasma glucagon-like peptide-1 levels. Upon HFD challenge, iKO mice had reduced metabolic efficiency apparent through decreased weight gain despite higher food intake. Concomitantly, energy expenditure was increased, and glucose tolerance was improved in HFD-fed iKO mice.

CONCLUSION

Our results indicate that deletion of intestinal SCD1 has significant impacts on bile acid homeostasis and whole-body energy balance, likely via activation of TGR5.

Intestinal stearoyl-CoA desaturase-1 regulates energy balance via alterations in bile acid homeostasis

Background and Aims

Stearoyl-CoA desaturase-1 (SCD1) converts saturated fatty acids into monounsaturated fatty acids and plays an important regulatory role in lipid metabolism. Previous studies have demonstrated that mice deficient in SCD1 are protected from diet-induced obesity and hepatic steatosis due to altered lipid esterification and increased energy expenditure. Previous studies in our lab have shown that intestinal SCD1 modulates intestinal and plasma lipids and alters cholesterol metabolism. Here we investigated a novel role for intestinal SCD1 in the regulation of systemic energy balance.

Methods

To interrogate the role of intestinal SCD1 in modulating whole body metabolism, intestine-specific Scd1 knockout (iKO) mice were maintained on standard chow diet or challenged with a high-fat diet (HFD). Studies included analyses of bile acid content and composition, metabolic phenotyping including body composition, indirect calorimetry, glucose tolerance analyses, and assessment of bile acid signaling pathways.

Results

iKO mice displayed elevated plasma and hepatic bile acid content and decreased fecal bile acid excretion, associated with increased expression of the ileal bile acid uptake transporter, Asbt . These increases were associated with increased expression of TGR5 targets, including Dio2 in brown adipose tissue and elevated plasma glucagon-like peptide-1 levels. Upon HFD challenge, iKO mice had reduced metabolic efficiency apparent through decreased weight gain despite higher food intake. Concomitantly, energy expenditure was increased, and glucose tolerance was improved in HFD-fed iKO mice.

Conclusion

Our results indicate that deletion of intestinal SCD1 has significant impacts on bile acid metabolism and whole-body energy balance, likely via activation of TGR5.

Bile salt signaling and bile salt-based therapies in cardiometabolic disease

Bile salts have an established role in the emulsification and intestinal absorption of dietary lipids, and their homeostasis is tightly controlled by various transporters and regulators in the enterohepatic circulation. Notably, emerging evidence points toward bile salts as major modulators of cardiometabolic disease (CMD), an umbrella disease of disorders affecting the heart and blood vessels that is caused by systemic metabolic diseases such as Type 2 diabetes mellitus (T2DM) and metabolic dysfunction-associated steatotic liver disease (MASLD), the latter encompassing also metabolic dysfunction-associated steatohepatitis (MASH). The underlying mechanisms of protective effects of bile salts are their hormonal properties, enabling them to exert versatile metabolic effects by activating various bile salt-responsive signaling receptors with the nuclear farnesoid X receptor (FXR) and the Takeda G-protein-coupled receptor 5 (TGR5) as most extensively investigated. Activation of FXR and TGR5 is involved in the regulation of glucose, lipid and energy metabolism, and inflammation. Bile salt-based therapies directly targeting FXR and TGR5 signaling have been evaluated for their therapeutic potential in CMD. More recently, therapeutics targeting bile salt transporters thereby modulating bile salt localization, dynamics, and signaling, have been developed and evaluated in CMD. Here, we discuss the current knowledge on the contribution of bile salt signaling in the pathogenesis of CMD and the potential of bile salt-based therapies for the treatment of CMD.

Trust Your Gut: Strategies and Tactics for Intestinally Restricted Drugs

Non-absorbable small-molecule drugs targeted to the gut represent an alternative approach to safe, non-systemic therapeutics. Such drugs remain confined to the gastrointestinal tract upon oral dosing by virtue of their limited passive permeability, increasing the local concentration at the site of action while minimizing exposure elsewhere in the body. Herein we review the latest advances in the field of gut-restricted therapeutics, highlighting the different strategies and tactics that medicinal chemists have employed in pursuit of drugs with minimal intestinal absorption.

Targeting bile acid signaling for the treatment of liver diseases: From bench to bed

Liver diseases and related complications have become one of the leading causes of morbidity and mortality worldwide, yet effective medicine or approved treatment approach is still limited. Thus, novel therapy is urgently required to prevent or at least slow down the growing burden of liver transplantation or even death caused by malignant liver diseases. As the irreplaceable modulator of hepatic and intestinal signaling cascades, bile acids (BAs) play complex physiological as well as pathological roles in regulating energy and immune homeostasis in various liver diseases, including but not limited to metabolic diseases and cholangiopathies, making them highly attractive therapeutic targets. In the current review, recent progress in the research of enterohepatic circulation of BAs and potential therapeutic targets of BAs signaling, especially the development of currently available treatments, including agonizts of FXR and TGR5, analogs of FGF19, inhibitors of ASBT, and the regulation of gut microbiome through fecal microbiota transplantation were extensively summarized. Their protective effects, molecular mechanisms, and outcomes of clinical trials were highlighted. The structural features of these candidates and perspectives for their future development were further discussed. In conclusion, we believe that pharmacological therapies targeting BAs signaling represent promising and efficient strategies for the treatment of complex and multifactorial liver disorders.

A Recent Ten-Year Perspective: Bile Acid Metabolism and Signaling

Bile acids are important physiological agents required for the absorption, distribution, metabolism, and excretion of nutrients. In addition, bile acids act as sensors of intestinal contents, which are determined by the change in the spectrum of bile acids during microbial transformation, as well as by gradual intestinal absorption. Entering the liver through the portal vein, bile acids regulate the activity of nuclear receptors, modify metabolic processes and the rate of formation of new bile acids from cholesterol, and also, in all likelihood, can significantly affect the detoxification of xenobiotics. Bile acids not absorbed by the liver can interact with a variety of cellular recipes in extrahepatic tissues. This provides review information on the synthesis of bile acids in various parts of the digestive tract, its regulation, and the physiological role of bile acids. Moreover, the present study describes the involvement of bile acids in micelle formation, the mechanism of intestinal absorption, and the influence of the intestinal microbiota on this process.

[The inhibitors of the apical sodium-dependent bile acid transporter (ASBT) as promising drugs]

Inhibition of the apical sodium-dependent bile acid transporter (ASBT, also known as IBAT - ileal bile acid transporter, SLC10A2) leads to disruption of the enterohepatic circulation of bile acids and their excretion with fecal masses. This is accompanied by cholesterol utilization for synthesis of new bile acids. ASBT inhibitors are promising drugs for the treatment of such diseases as non-alcoholic fatty liver disease, non-alcoholic steatohepatitis, type 2 diabetes mellitus, necrotic enterocolitis, chronic constipation, atherosclerosis. To date the most known chemically synthesized inhibitors are: A3309, SHP626, A4250, 264W94, GSK2330672, SC-435. All of them are at different stages of clinical trials, which confirm the high efficacy and good tolerance of these inhibitors. Current trends in this field also include directed chemical synthesis of ASBT inhibitors, as well as their search among substances of plant origin.

ASBT(SLC10A2): A promising target for treatment of diseases and drug discovery

Bile acids has gradually become a new focus in various diseases, and ASBT as a transporter responsible for the reabsorption of ileal bile acids, is a key hinge associated to the bile acids-cholesterol balance and bile acids of enterohepatic circulation. The cumulative studies have also shown that ASBT is a promising target for treatment of liver, gallbladder, intestinal and metabolic diseases. This article briefly reviewed the process of bile acids enterohepatic circulation, as well as the regulations of ASBT expression, covering transcription factors, nuclear receptors and gut microbiota. In addition, the relationship between ASBT and various diseases were discussed in this paper. According to the structural classification of ASBT inhibitors, the research status of ASBT inhibitors and potential ASBT inhibitors of traditional Chinese medicine (such resveratrol, jatrorrhizine in Coptis chinensis) were summarized. This review provides a basis for the development of ASBT inhibitors and the treatment strategy of related diseases.

Intestinal Absorption of Bile Acids in Health and Disease

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

Bile acids in glucose metabolism and insulin signalling - mechanisms and research needs

Of all the novel glucoregulatory molecules discovered in the past 20 years, bile acids (BAs) are notable for the fact that they were hiding in plain sight. BAs were well known for their requirement in dietary lipid absorption and biliary cholesterol secretion, due to their micelle-forming properties. However, it was not until 1999 that BAs were discovered to be endogenous ligands for the nuclear receptor FXR. Since that time, BAs have been shown to act through multiple receptors (PXR, VDR, TGR5 and S1PR2), as well as to have receptor-independent mechanisms (membrane dynamics, allosteric modulation of N-acyl phosphatidylethanolamine phospholipase D). We now also have an appreciation of the range of physiological, pathophysiological and therapeutic conditions in which endogenous BAs are altered, raising the possibility that BAs contribute to the effects of these conditions on glycaemia. In this Review, we highlight the mechanisms by which BAs regulate glucose homeostasis and the settings in which endogenous BAs are altered, and provide suggestions for future research.

A novel ASBT inhibitor, IMB17-15, repressed nonalcoholic fatty liver disease development in high-fat diet-fed Syrian golden hamsters

The manipulation of bile acid (BA) homeostasis by blocking the ileal apical Na⁺-dependent bile salt transporter (ASBT/SLC10A2) may have therapeutic effects in nonalcoholic fatty liver disease. We developed a novel ASBT inhibitor, an N-(3,4-o-dichlorophenyl)-2-(3-trifluoromethoxy) benzamide derivative referred to as IMB17-15, and investigated its therapeutic effects and the molecular mechanisms underlying the effects. Syrian golden hamsters were challenged with high-fat diet (HFD) to induce NAFLD and were subsequently administered 400 mg/kg IMB17-15 by gavage daily for 21 days. Serum, liver, and fecal samples were collected for further analysis. Plasma concentration-time profiles of IMB17-15 were also constructed. The human hepatocyte cell line HL-7702 was treated with Oleic acid (OA) with or without IMB17-15. Western blotting and real-time PCR were used to study the molecular mechanisms of IMB17-15. We found that IMB17-15 inhibited ASBT and subsequently suppressed ileal farnesoid X receptor (FXR) and FXR-activated fibroblast growth factor15/19 (FGF15/19) expression, which reduced the hepatic phosphorylated extracellular regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) levels and upregulated the cholesterol 7α-hydroxylase (CYP7A1) activity. Additionally, IMB17-15 stimulated adenosine monophosphate (AMP)-activated protein kinase (AMPKα) phosphorylation and enhanced peroxisome proliferator activated receptor α (PPARα) expression and thus promoted triglyceride (TG) oxidation and high-density lipoprotein cholesterol (HDL-c) metabolism through an ASBT-independent mechanism. In conclusion, a novel ASBT inhibitor known as IMB17-15 protected hamsters against HFD-induced NFALD by manipulating BA and lipid homeostasis. IMB17-15 also reduced lipid deposition in human hepatic cell lines, indicating that it may be useful as a therapy for NAFLD patients.

New therapies target the toxic consequences of cholestatic liver disease

In most cholestatic liver diseases the primary cholestasis-causing lesions are located in the biliary tree and may be of (auto)immune origin. Bile salts are responsible for the secondary toxic consequences. Bile salt and nuclear hormone directed therapies primarily aim at improving this secondary toxic injury. In primary biliary cholangitis, trials show statistically significant responses on biochemical endpoints. Preclinical studies suggest that FXR- and PPAR-agonists, inhibitors of the apical sodium-dependent bile salt transporter (ASBT-inhibitors) and the C₂₃ UDCA derivative nor-UDCA are promising agents for the treatment of primary sclerosing cholangitis (PSC). Area covered: Pharmaceuticals that interfere with bile salt signaling in humans for the treatment of chronic cholestatic liver disease are reviewed. Expert commentary: Nuclear hormone receptors, bile salt transport proteins and receptors provide targets for novel therapies of cholestatic liver disease. These drugs show positive results on biochemical endpoints. For histological endpoints, survival and transplant-free survival, long-term trials are needed. For relief of symptoms, such as fatigue and pruritus, these drugs have yet to prove their value.

Increased Bile Acid Synthesis and Deconjugation After Biliopancreatic Diversion

Biliopancreatic diversion (BPD) improves insulin sensitivity and decreases serum cholesterol out of proportion with weight loss. Mechanisms of these effects are unknown. One set of proposed contributors to metabolic improvements after bariatric surgeries is bile acids (BAs). We investigated the early and late effects of BPD on plasma BA levels, composition, and markers of BA synthesis in 15 patients with type 2 diabetes (T2D). We compared these to the early and late effects of Roux-en-Y gastric bypass (RYGB) in 22 patients with T2D and 16 with normal glucose tolerance. Seven weeks after BPD, insulin sensitivity had doubled and serum cholesterol had halved. At this time, BA synthesis markers and total plasma BAs, particularly unconjugated BAs, had markedly risen; this effect could not be entirely explained by low FGF19. In contrast, after RYGB, insulin sensitivity improved gradually with weight loss and cholesterol levels declined marginally; BA synthesis markers were decreased at an early time point (2 weeks) after surgery and returned to the normal range 1 year later. These findings indicate that BA synthesis contributes to the decreased serum cholesterol after BPD. Moreover, they suggest a potential role for altered enterohepatic circulation of BAs in improving insulin sensitivity and cholesterol metabolism after BPD.

Design, Synthesis, and Therapeutic Evaluation of Poly(acrylic acid)-tetraDOCA Conjugate as a Bile Acid Transporter Inhibitor

Regulation of cholesterol and bile acid homeostasis has been attracting attention as a pharmaceutical target for the treatment of diseases, such as hypercholesterolaemia and type 2 diabetes. In recent years, small bile acid analogues have been developed for the purpose of apical sodium-dependent bile acid transporter (ASBT) inhibition. Here, we designed a novel hydrophilic ASBT inhibitor using oligomeric bile acid with a high affinity with ASBT. Polyacrylic acid-tetraDOCA conjugates (PATD) have the ability to bind to ASBT in order to induce hypocholesterolemic effects. Both the viability and the functionality of PATD were evaluated in vitro, showing that PATDs were effective in inhibiting the increases of cholesterol in the blood and oil in the liver induced by high fat diet (HFD). The results indicated that the newly developed biomaterials with oligomeric bile acids and a hydrophilic polymer are potent therapeutic agents for hyperlipidemia.

Metabolic effects of intestinal absorption and enterohepatic cycling of bile acids

The classical functions of bile acids include acting as detergents to facilitate the digestion and absorption of nutrients in the gut. In addition, bile acids also act as signaling molecules to regulate glucose homeostasis, lipid metabolism and energy expenditure. The signaling potential of bile acids in compartments such as the systemic circulation is regulated in part by an efficient enterohepatic circulation that functions to conserve and channel the pool of bile acids within the intestinal and hepatobiliary compartments. Changes in hepatobiliary and intestinal bile acid transport can alter the composition, size, and distribution of the bile acid pool. These alterations in turn can have significant effects on bile acid signaling and their downstream metabolic targets. This review discusses recent advances in our understanding of the inter-relationship between the enterohepatic cycling of bile acids and the metabolic consequences of signaling via bile acid-activated receptors, such as farnesoid X nuclear receptor (FXR) and the G-protein-coupled bile acid receptor (TGR5).

Intestinal transport and metabolism of bile acids

In addition to their classical roles as detergents to aid in the process of digestion, bile acids have been identified as important signaling molecules that function through various nuclear and G protein-coupled receptors to regulate a myriad of cellular and molecular functions across both metabolic and nonmetabolic pathways. Signaling via these pathways will vary depending on the tissue and the concentration and chemical structure of the bile acid species. Important determinants of the size and composition of the bile acid pool are their efficient enterohepatic recirculation, their host and microbial metabolism, and the homeostatic feedback mechanisms connecting hepatocytes, enterocytes, and the luminal microbiota. This review focuses on the mammalian intestine, discussing the physiology of bile acid transport, the metabolism of bile acids in the gut, and new developments in our understanding of how intestinal metabolism, particularly by the gut microbiota, affects bile acid signaling.

c-Fos mediates repression of the apical sodium-dependent bile acid transporter by fibroblast growth factor-19 in mice

Fibroblast growth factor-19 (FGF-19), a bile acid-responsive enterokine, is secreted by the ileum and regulates a variety of metabolic processes. These studies examined the signal transduction pathways operant in FGF-19-mediated repression of the apical sodium-dependent bile acid transporter (ASBT). Responses to FGF-19 were assessed in Caco-2 and CT-26 cells and in mice where c-fos was conditionally silenced in the intestine by a cre-lox strategy. FGF-19 treatment of Caco-2 cells or wild-type mice led to a significant reduction in ASBT protein expression and enhanced phosphorylation of extracellular signaling kinase 1/2 (ERK1/2), c-Fos, and c-Jun. FGF-19 treatment of Caco-2 cells led to a reduction in activity of the human ASBT promoter and this repression could be blocked by treatment with a mitogen-activated protein kinase/ERK kinase (MEK1/2) inhibitor or by silencing jun kinase 1, jun kinase 2, c-fos, or c-jun. Site directed mutagenesis of a c-fos binding element in the ASBT promoter blocked FGF-19-mediated repression in luciferase reporter constructs. ASBT promoter activity was repressed by FGF-19 in CT-26 cells and this repression could be reduced by MEK1/2 inhibition or silencing c-fos. FGF-19-mediated repression of ASBT protein expression was abrogated in mice where c-fos was conditionally silenced in the intestine. In contrast, ASBT was repressed in the c-Fos expressing gallbladders of the same mice. The studies demonstrate that FGF-19 represses the expression of ASBT in the ileum and gallbladder via a signal transduction pathway involving MEK1/2, ERK1/2, JNK1, JNK2, and c-Fos.

The solute carrier family 10 (SLC10): beyond bile acid transport

The solute carrier (SLC) family 10 (SLC10) comprises influx transporters of bile acids, steroidal hormones, various drugs, and several other substrates. Because the seminal transporters of this family, namely, sodium/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2), were primarily bile acid transporters, the term "sodium bile salt cotransporting family" was used for the SLC10 family. However, this notion became obsolete with the finding of other SLC10 members that do not transport bile acids. For example, the sodium-dependent organic anion transporter (SOAT; SLC10A6) transports primarily sulfated steroids. Moreover, NTCP was shown to also transport steroids and xenobiotics, including HMG-CoA inhibitors (statins). The SLC10 family contains four additional members, namely, P3 (SLC10A3; SLC10A3), P4 (SLC10A4; SLC10A4), P5 (SLC10A5; SLC10A5) and SLC10A7 (SLC10A7), several of which were unknown or considered hypothetical until approximately a decade ago. While their substrate specificity remains undetermined, great progress has been made towards their characterization in recent years. Explicitly, SLC10A4 may participate in vesicular storage or exocytosis of neurotransmitters or mastocyte mediators, whereas SLC10A5 and SLC10A7 may be involved in solute transport and SLC10A3 may have a role as a housekeeping protein. Finally, the newly found role of bile acids in glucose and energy homeostasis, via the TGR5 receptor, sheds new light on the clinical relevance of ASBT and NTCP. The present mini-review provides a brief summary of recent progress on members of the SLC10 family.

Inhibition of ileal apical but not basolateral bile acid transport reduces atherosclerosis in apoE⁻/⁻ mice

OBJECTIVE

Interruption of the enterohepatic circulation of bile acids induces hepatic bile acid synthesis, increases hepatic cholesterol demand, and increases clearance of apoB-containing lipoproteins in plasma. Based on these effects, bile acid sequestrants have been used for many years to treat hypercholesterolemia and the associated atherosclerosis. The objective of this study was to determine the effect of blocking ileal apical versus basolateral membrane bile acid transport on the development of hypercholesterolemia and atherosclerosis in mouse models.

METHODS AND RESULTS

ApoE(-/-) and Ldlr(-/-) mice deficient in the apical sodium-dependent bile acid transporter (Asbt) or apoE(-/-) mice deficient in the basolateral bile acid transporter (Ostα) were fed an atherogenic diet for 16 weeks. Bile acid metabolism, cholesterol metabolism, gene expression, and development of atherosclerosis were examined. Mice deficient in Asbt exhibited the classic response to interruption of the enterohepatic circulation of bile acids, including significant reductions in hepatic and plasma cholesterol levels, and reduced aortic cholesteryl ester content. Ileal Fibroblast Growth Factor-15 (FGF15) expression was significantly reduced in Asbt(-/-)apoE(-/-) mice and was inversely correlated with expression of hepatic cholesterol 7-hydroxylase (Cyp7a1). Ileal FGF15 expression was directly correlated with plasma cholesterol levels and aortic cholesterol content. In contrast, plasma and hepatic cholesterol levels and atherosclerosis development were not reduced in apoE(-/-) mice deficient in Ostα.

CONCLUSIONS

Decreases in ileal FGF15, with subsequent increases in hepatic Cyp7a1 expression and bile acid synthesis appear to be necessary for the plasma cholesterol-lowering and atheroprotective effects associated with blocking intestinal bile acid absorption.

Discovery of a highly potent, nonabsorbable apical sodium-dependent bile acid transporter inhibitor (GSK2330672) for treatment of type 2 diabetes

The apical sodium-dependent bile acid transporter (ASBT) transports bile salts from the lumen of the gastrointestinal (GI) tract to the liver via the portal vein. Multiple pharmaceutical companies have exploited the physiological link between ASBT and hepatic cholesterol metabolism, which led to the clinical investigation of ASBT inhibitors as lipid-lowering agents. While modest lipid effects were demonstrated, the potential utility of ASBT inhibitors for treatment of type 2 diabetes has been relatively unexplored. We initiated a lead optimization effort that focused on the identification of a potent, nonabsorbable ASBT inhibitor starting from the first-generation inhibitor 264W94 (1). Extensive SAR studies culminated in the discovery of GSK2330672 (56) as a highly potent, nonabsorbable ASBT inhibitor which lowers glucose in an animal model of type 2 diabetes and shows excellent developability properties for evaluating the potential therapeutic utility of a nonabsorbable ASBT inhibitor for treatment of patients with type 2 diabetes.

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.

Inhibition of apical sodium-dependent bile acid transporter as a novel treatment for diabetes

Bile acids are recognized as metabolic modulators. The present study was aimed at evaluating the effects of a potent Asbt inhibitor (264W94), which blocks intestinal absorption of bile acids, on glucose homeostasis in Zucker Diabetic Fatty (ZDF) rats. Oral administration of 264W94 for two wk increased fecal bile acid concentrations and elevated non-fasting plasma total Glp-1. Treatment of 264W94 significantly decreased HbA1c and glucose, and prevented the drop of insulin levels typical of ZDF rats in a dose-dependent manner. An oral glucose tolerance test revealed up to two-fold increase in plasma total Glp-1 and three-fold increase in insulin in 264W94 treated ZDF rats at doses sufficient to achieve glycemic control. Tissue mRNA analysis indicated a decrease in farnesoid X receptor (Fxr) activation in small intestines and the liver but co-administration of a Fxr agonist (GW4064) did not attenuate 264W94 induced glucose lowering effects. In summary, our results demonstrate that inhibition of Asbt increases bile acids in the distal intestine, promotes Glp-1 release and may offer a new therapeutic strategy for type 2 diabetes mellitus.

Barley intake induces bile acid excretion by reduced expression of intestinal ASBT and NPC1L1 in C57BL/6J mice

To investigate the hypocholesterolemic mechanism of barley in vivo, six-week-old C57BL/6J mice were fed a high-fat diet (HFD) or high-fat diet containing barley (HFD-B) for seven weeks. Total and LDL cholesterol concentrations were significantly reduced in the HFD-B group while fecal cholesterol and bile acid was increased. Real-time PCR and immunoblot analysis revealed the induction of FXR expression, which in turn suppressed the expression of ASBT and NPC1L1 in the HFD-B group compared with the controls. In the liver, the expression of HMG-CoA reductase was significantly reduced while LDL receptor expression was unaltered in the HFD-B group compared with the controls. Our data suggest that the hypocholesterolemic effects of barley are primarily the result of reduced dietary cholesterol uptake and bile acid resorption. Reduced expression of intestinal ASBT and NPC1L1 may play a key role in the regulation of dietary cholesterol and bile acid metabolism in mice consuming a diet containing barley.

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

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

Bile acid transporters

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

Bile acid transporters in health and disease

In recent years the discovery of a number of major transporter proteins expressed in the liver and intestine specifically involved in bile acid transport has led to improved understanding of bile acid homeostasis and the enterohepatic circulation. Sodium (Na(+))-dependent bile acid uptake from portal blood into the liver is mediated primarily by the Na(+) taurocholate co-transporting polypeptide (NTCP), while secretion across the canalicular membrane into the bile is carried out by the bile salt export pump (BSEP). In the ileum, absorption of bile acids from the lumen into epithelial cells is mediated by the apical Na(+) bile salt transporter (ASBT), whereas exit into portal blood across the basolateral membrane is mediated by the organic solute transporter alpha/beta (OSTalpha/beta) heterodimer. Regulation of transporter gene expression and function occurs at several different levels: in the nucleus, members of the nuclear receptor superfamily, regulated by bile acids and other ligands are primarily involved in controlling gene expression, while cell signalling events directly affect transporter function, and subcellular localization. Polymorphisms, dysfunction, and impaired adaptive responses of several of the bile acid transporters, e.g. BSEP and ASBT, results in liver and intestinal disease. Bile acid transporters are now understood to play central roles in driving bile flow, as well as adaptation to various pathological conditions, with complex regulation of activity and function in the nucleus, cytoplasm, and membrane.

Novel non-systemic inhibitor of ileal apical Na+-dependent bile acid transporter reduces serum cholesterol levels in hamsters and monkeys

1-{7-[(1-(3,5-Diethoxyphenyl)-3-{[(3,5-difluorophenyl)(ethyl)amino]carbonyl}-4-oxo-1,4-dihydroquinolin-7-yl)oxy]heptyl}-1-methylpiperidinium bromide, R-146224, is a potent, specific ileum apical sodium-dependent bile acid transporter (ASBT) inhibitor; concentrations required for 50% inhibition of [3H]taurocholate uptake in human ASBT-expressing HEK-293 cells and hamster ileum tissues were 0.023 and 0.73 microM, respectively. In bile-fistula rats, biliary and urinary excretion 48 h after 10 mg/kg [14C]R-146224, were 1.49+/-1.75% and 0.14+/-0.05%, respectively, demonstrating extremely low absorption. In hamsters, R-146224 dose-dependently reduced gallbladder bile [3H]taurocholate uptake (ED50: 2.8 mg/kg). In basal diet-fed hamsters, 14-day 30-100 mg/kg R-146224 dose-dependently reduced serum total cholesterol (approximately 40%), high density lipoprotein (HDL) cholesterol (approximately 37%), non-HDL cholesterols (approximately 20%), and phospholipids (approximately 20%), without affecting serum triglycerides, associated with reduced free and esterified liver cholesterol contents. In normocholesterolemic cynomolgus monkeys, R-146224 specifically reduced non-HDL cholesterol. In human ileum specimens, R-146224 dose-dependently inhibited [3H]taurocholate uptake. Potent non-systemic ASBT inhibitor R-146224 decreases bile acid reabsorption by inhibiting the ileal bile acid active transport system, resulting in hypolipidemic activity.

The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships

The solute carrier family 10 (SLC10) comprises two sodium-dependent bile acid transporters, i.e. the Na(+)/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2). These carriers are essentially involved in the maintenance of the enterohepatic circulation of bile acids mediating the first step of active bile acid transport through the membrane barriers in the liver (NTCP) and intestine (ASBT). Recently, four new members of the SLC10 family were described and referred to as P3 (SLC10A3), P4 (SLC10A4), P5 (SLC10A5) and sodium-dependent organic anion transporter (SOAT; SLC10A6). Experimental data supporting carrier function of P3, P4, and P5 is currently not available. However, as demonstrated for SOAT, not all members of the SLC10 family are bile acid transporters. SOAT specifically transports steroid sulfates such as oestrone-3-sulfate and dehydroepiandrosterone sulfate in a sodium-dependent manner, and is considered to play an important role for the cellular delivery of these prohormones in testes, placenta, adrenal gland and probably other peripheral tissues. ASBT and SOAT are the most homologous members of the SLC10 family, with high sequence similarity ( approximately 70%) and almost identical gene structures. Phylogenetic analyses of the SLC10 family revealed that ASBT and SOAT genes emerged from a common ancestor gene. Structure-activity relationships of NTCP, ASBT and SOAT are discussed at the amino acid sequence level. Based on the high structural homology between ASBT and SOAT, pharmacological inhibitors of the ASBT, which are currently being tested in clinical trials for cholesterol-lowering therapy, should be evaluated for their cross-reactivity with SOAT.

Inhibitory effects of choleretic hydroxyacetophenones on ileal bile acid transport in rats

The effects of the choleretic and cholesterol lowering compound, 2,4,6-trihydroxyacetophenone (THA) and its analog, 2,6-dihydroxyacetophenone (DHA), on ileal bile acid absorption were investigated in rats. THA inhibited taurocholate (TC) uptake into ileal brush-border membrane vesicles (BBMV), showing a maximum inhibition of 50%, whereas DHA completely inhibited TC uptake into ileal BBMV. THA exhibited competitive inhibition with a Ki of 9.88 mM, while DHA showed non-competitive inhibition with a Ki of 7.65 mM. Both total and ouabain-sensitive basolateral membrane (BLM) Na+-K+-ATPase activities, which are essential for maintenance of the Na+-gradient for bile acid transport, were inhibited by THA and DHA in a dose-dependent manner. The inhibition of BLM ATPase was uncompetitive with a Ki of 10.1 and 5.0 mM for THA and DHA, respectively. Administration of THA or DHA (400 micromol/kg) twice a day, to hypercholesterolemic rats for 3 weeks caused similar and marked reductions in plasma cholesterol to 60% of the cholesterol-fed controls. The data suggest that the inhibitory actions of THA and DHA on two essential components of ileal bile acid recycling to liver could, in part, contribute to the cholesterol lowering effect of the hydroxyacetophenone compounds. These effects on decreasing bile acid recycling, in combination with their potent choleretic effect, accelerating biliary excretion of bile acids, are responsible for the effective cholesterol lowering capacities of these compounds.

SC-435, an ileal apical sodium-codependent bile acid transporter inhibitor alters mRNA levels and enzyme activities of selected genes involved in hepatic cholesterol and lipoprotein metabolism in guinea pigs

We have demonstrated that SC-435, an apical sodium codependent bile acid transporter (ASBT) inhibitor, lowers plasma low-density lipoprotein cholesterol (LDL-C) concentrations in guinea pigs. The purpose of this study was to further examine the hypocholesterolemic effects of SC-435, by measuring the activity and RNA expression of regulatory enzymes of hepatic cholesterol and lipoprotein metabolism. In addition, the use of a combination (COMBO) therapy with simvastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor, was also tested. Male Hartley guinea pigs were randomly allocated to one of three diets (n=10 per group), for 12 weeks. The control diet contained no ASBT inhibitor or simvastatin. The monotherapy diet (ASBTi) contained 0.1% of SC-435. The COMBO therapy consisted of a lower dose of SC-435 (0.03%) and 0.05% simvastatin. Cholesterol ester transfer protein (CETP) and HMG-CoA reductase mRNA abundance were determined using RT-PCR techniques. Hepatic HMG-CoA reductase and cholesterol 7alpha-hydroxylase (CYP7) activities were measured by radioisotopic methods. Compared to the control group, CETP activity was 34% and 56% lower with ASBTi and COMBO, respectively. Similarly, CETP mRNA expression was reduced by 36% and 73% in ASBTi and COMBO groups, respectively. Cholesterol 7alpha-hydroxylase and HMG-CoA reductase activities were increased approximately 2-fold with ASBTi and COMBO treatments, respectively. Likewise, HMG-CoA reductase mRNA expression was increased 33% with ASBTi treatment. These results suggest that both SC-435 monotherapy and combination therapy lower LDL cholesterol concentrations by altering both hepatic cholesterol homeostasis and the intravascular processing of lipoproteins in guinea pigs.

Molecular mechanisms of cholesterol absorption and transport in the intestine

Many enzymes and transport proteins participate in cholesterol absorption. This review summarizes recent results on several proteins that are important for each step of the cholesterol absorption pathway, including the important roles of: (i) pancreatic triglyceride lipase (PTL), carboxyl ester lipase (CEL), and ileal bile acid transporter in determining the rate of cholesterol absorption; (ii) ATP binding cassette (ABC) transporters and the Niemann-Pick C-1 like-1 (NPC1L1) protein as intestinal membrane gatekeepers for cholesterol efflux and influx; and (iii) intracellular membrane vesicles and transport proteins in lipid trafficking through intracellular compartments prior to lipoprotein assembly and secretion to plasma circulation.

Inhibition of ileal bile acid transport lowers plasma cholesterol levels by inactivating hepatic farnesoid X receptor and stimulating cholesterol 7 alpha-hydroxylase

We investigated the effect of SC-435, a competitive inhibitor of ileal apical sodium-dependent bile acid cotransporter (ASBT) on ileal bile acid absorption and the hepatic nuclear receptor FXR (farnesoid X receptor), which regulates cholesterol 7 alpha-hydroxylase (CYP7A1) activity and mRNA levels. Eighteen New Zealand White (NZW) rabbits were divided into 2 groups: controls (n = 10) and fed SC-435 125 mg/kg/d for 1 week (n = 8). In rabbits treated with SC-435, fecal bile acid outputs increased by more than 8 times, reflecting substantial bile acid malabsorption. Plasma cholesterol levels decreased 26%, while bile acid pool sizes and biliary bile acid outputs did not change after treatment. CYP7A1 activity increased 64% and mRNA rose by 4 times after treatment. The expression of FXR target genes in the liver, short heterodimer partner (SHP) and bile salt export pump (BSEP), decreased 11.6 and 2.6 times, respectively, after treatment, which indicates inactivation of hepatic FXR. However, the mRNA levels of ileal bile acid binding protein (IBABP) did not change significantly, while ileal ASBT mRNA expression increased by 2.4 times after treatment. Rabbits treated with SC-435 developed ileal bile acid malabsorption, which decreased the return of bile acids (FXR ligands) to the liver to inactivate hepatic FXR, which upregulated CYP7A1 and lowered plasma cholesterol levels. Although fecal bile acid malabsorption was substantial, increased bile acid production from hepatic cholesterol kept biliary bile acid outputs intact. Thus, a new balance was reached in the liver, where increased bile acid synthesis compensated for diminished ileal bile acid absorption to maintain the circulating enterohepatic bile acid pool.

SC-435, an ileal apical sodium co-dependent bile acid transporter (ASBT) inhibitor lowers plasma cholesterol and reduces atherosclerosis in guinea pigs

Male Hartley guinea pigs were randomly allocated to one of four treatments, 10 guinea pigs per group, for 12 weeks. The control diet contained no ASBT inhibitor (ASBTi) or simvastatin. Low ASBTi (LowASBTi) and high ASBTi (HighASBTi) were monotherapies containing 0.03 g/100 g and 0.1 g/100 g of the ASBTi SC-435. Combination therapy (COMBO) was a combination therapy consisting of 0.03 g/100 g ASBTi and 0.05 g/100 g simvastatin. Based on food consumption, guinea pigs received 17.2 and 47.8 mg/kg per day ASBTi in the ASBTi groups or 13.7 mg/kg per day ASBTi and 21.4 mg/kg per day simvastatin in the COMBO group. The amount of cholesterol in each diet was 0.25 g/100 g. LDL cholesterol was 40 and 70% lower with the HighASBTi and COMBO treatments compared to controls. Plasma triglycerides (TG) were 70% lower with COMBO therapy while HDL cholesterol was 43-47% higher with all treatments. Hepatic free cholesterol was reduced 60-80% with all treatments. Cholesterol content in the aortic arch was reduced by 25 and 42% in the HighASBTi and COMBO groups. Fecal bile acids were increased by 2.5- and 4-fold with HighASBTi and COMBO treatments. These data suggest that the interruption in the enterohepatic circulation of bile acids by ASBTi and statin co-administration therapy cause a significant reduction in plasma cholesterol concentrations and attenuate the progression of atherosclerosis in guinea pigs.

Bile acid regulation of hepatic physiology: III. Regulation of bile acid synthesis: past progress and future challenges

Bile acids, amphipathic detergent-like molecules synthesized from cholesterol, are highly conserved by means of enterohepatic circulation. They participate in the generation of bile flow and biliary lipid secretion and also promote absorption of fat-soluble vitamins and lipids. Conversion of cholesterol to bile acids represents a quantitatively important route to eliminate cholesterol from the body. Regulation of bile acid synthesis involves a complex and interrelated group of transcription regulators that link bile acid synthesis to cholesterol and fatty acid metabolism. Targeting key steps of bile acid synthetic pathways as well as the metabolic network that maintains homeostatic levels of lipids should provide exciting novel opportunities for the treatment of cardiovascular and liver diseases.

Monitoring hepatic cholesterol 7alpha-hydroxylase activity by assay of the stable bile acid intermediate 7alpha-hydroxy-4-cholesten-3-one in peripheral blood

We describe an accurate method for monitoring the enzymatic activity of hepatic cholesterol 7alpha-hydroxylase (C7alphaOH; CYP7A1), the rate-limiting and major regulatory enzyme in the synthesis of bile acids. Assay of 7alpha-hydroxy-4-cholesten-3-one (C4), an intermediate in bile acid synthesis, revealed that the level of C4 in peripheral blood serum or plasma showed a strong correlation to the enzymatic activity of hepatic C7alphaOH, both at steady-state conditions (r = 0.929) as well as during the rapid changes that occur during the diurnal phases. This assay should be of value in clarifying the regulation of bile acid synthesis in vivo in laboratory animals and humans since it allows for the monitoring of hepatic C7alphaOH activity using peripheral blood samples.

Pharmacological interference with intestinal bile acid transport reduces plasma cholesterol in LDL receptor/apoE deficiency

Reduction of plasma cholesterol by statins is fundamental to prevent coronary heart disease. Such therapy is often sub-optimal, however, particularly in patients with reduced LDL receptors (familial hypercholesterolemia), and novel or adjuvant therapies are therefore warranted. Cholesterol elimination is profoundly influenced by the rate of its conversion to bile acids (BA), regulated by the enzyme Cyp7a1. Induced fecal loss of BA by resin treatment reduces plasma cholesterol, presumably through induction of hepatic LDL receptors (LDLR). We here describe the effect of PR835, a drug belonging to a new class of lipid-lowering agents that inhibit the Slc10a2 protein, the intestinal transporter responsible for active uptake of BA. Treatment reduced plasma cholesterol by 40% in mice devoid of both the LDLR and its ligand, apoE, while triglycerides and HDL cholesterol were unchanged. Cyp7a1 enzyme activity and mRNA were induced several-fold, and hepatic HMG CoA reductase mRNA increased, mirroring an induced synthesis of BA and cholesterol. The addition of a statin potentiated the effect, leading to reductions of plasma total and LDL cholesterol by 64% and 70%, respectively. These effects could not be attributed to induction of other known hepatic lipoprotein receptors and indicate the presence of new points of targeting in lipid-lowering therapy.