Modulation of ileal bile acid transporter (ASBT) activity by depletion of plasma membrane cholesterol: association with lipid rafts

Oligomerization of drug transporters: Forms, functions, and mechanisms

Drug transporters are essential players in the transmembrane transport of a wide variety of clinical drugs. The broad substrate spectra and versatile distribution pattern of these membrane proteins infer their pharmacological and clinical significance. With our accumulating knowledge on the three-dimensional structure of drug transporters, their oligomerization status has become a topic of intense study due to the possible functional roles carried out by such kind of post-translational modification (PTM). In-depth studies of oligomeric complexes formed among drug transporters as well as their interactions with other regulatory proteins can help us better understand the regulatory mechanisms of these membrane proteins, provide clues for the development of novel drugs, and improve the therapeutic efficacy. In this review, we describe different oligomerization forms as well as their structural basis of major drug transporters in the ATP-binding cassette and solute carrier superfamilies, summarize our current knowledge on the influence of oligomerization for protein expression level and transport function of these membrane proteins, and discuss the regulatory mechanisms of oligomerization. Finally, we highlight the challenges associated with the current oligomerization studies and propose some thoughts on the pharmaceutical application of this important drug transporter PTM.

Enhanced Oral Absorption and Liver Distribution of Polymeric Nanoparticles through Traveling the Enterohepatic Circulation Pathways of Bile Acid

The intestinal epithelium is known to be a main hindrance to oral delivery of nanoparticles. Even though surface ligand modification can enhance cellular uptake of nanoparticles, the "easy entry and hard across" was frequently observed for many active targeting nanoparticles. Here, we fabricated polymeric nanoparticles relayed by bile acid transporters with monomethoxy poly(ethylene glycol)-poly(D,l-lactide) and deoxycholic acid-conjugated poly(2-ethyl-2-oxazoline)-poly(D,l-lactide) based on structural characteristics of intestine epithelium and the absorption characteristics of endogenous substances. As anticipated, deoxycholic acid-modified polymeric nanoparticles featuring good stability in simulated gastrointestinal fluid could notably promote the internalization of their payload by Caco-2 cells through mediation of apical sodium-dependent bile acid transporter (ASBT) and transmembrane transport of the nanoparticles across Caco-2 cell monolayers via relay-guide of ASBT, ileal bile acid-binding protein, and the heteromeric organic solute transporter (OSTα-OSTβ) along with multidrug resistance-associated protein 3 (MRP3) evidenced by competitive inhibition and fluorescence immunoassay, which was further visually confirmed by the stronger fluorescence from C6-labeled nanoparticles inside enterocytes and the basal side of the intestinal epithelium of mice. The transcellular transport of deoxycholic acid-modified nanoparticles in an intact form was mediated by caveolin/lipid rafts and clathrin with intracellular trafficking trace of endosome-lysosome-ER-Golgi apparatus and bile acid transport route. Furthermore, the increased uptake by HepG2 cells compared with unmodified nanoparticles evidenced the target ability of deoxycholic acid-modified nanoparticles to the liver, which was further supported by ex vivo imaging of excised major organs of mice. Thus, this study provided a feasible and potential strategy to further enhance transepithelial transport efficiency and liver-targeted ability of nanoparticles by means of the specific enterohepatic circulation pathways of bile acid.

Free Cholesterol Affects the Function and Localization of Human Na+/Taurocholate Cotransporting Polypeptide (NTCP) and Organic Cation Transporter 1 (OCT1)

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are associated with obesity. They are accompanied by increased levels of free cholesterol in the liver. Most free cholesterol resides within the plasma membrane. We assessed the impact of adding or removing free cholesterol on the function and localization of two hepatocellular uptake transporters: the Na⁺/taurocholate cotransporting polypeptide (NTCP) and the organic cation transporter 1 (OCT1). We used a cholesterol-MCD complex (cholesterol) to add cholesterol and methyl-β-cyclodextrin (MCD) to remove cholesterol. Our results demonstrate that adding cholesterol decreases NTCP capacity from 132 ± 20 to 69 ± 37 µL/mg/min and OCT1 capacity from 209 ± 66 to 125 ± 26 µL/mg/min. Removing cholesterol increased NTCP and OCT1 capacity to 224 ± 65 and 279 ± 20 µL/mg/min, respectively. In addition, adding cholesterol increased the localization of NTCP within lipid rafts, while adding or removing cholesterol increased OCT1 localization in lipid rafts. These results demonstrate that increased cholesterol levels can impair NTCP and OCT1 function, suggesting that the free cholesterol content of the liver can alter bile acid and drug uptake into the liver. This could explain the increased plasma bile acid levels in NAFLD and NASH patients and potentially lead to altered drug disposition.

Modulatory Effect of Theaflavins on Apical Sodium-Dependent Bile Acid Transporter (ASBT) Activity

Inhibiting apical sodium-dependent bile acid transporter (ASBT) has been identified as a potential strategy to reduce plasma cholesterol levels. Thus, in this study, we aimed to identify polyphenols that inhibited ASBT activity and to elucidate their mechanism. ASBT is responsible for most of the taurocholic acid (TC) uptake in Caco-2 cells. Of the 39 polyphenols examined, theaflavin (TF)-3-gallate (TF2A) and theaflavin-3'-gallate (TF2B) have been found to significantly reduce TC uptake in Caco-2 cells to 37.4 ± 2.8 and 33.8 ± 4.0%, respectively, of that in the untreated cells. The results from the TC uptake assay using N-acetylcysteine suggested that the inhibitory effect of TF2A and TF2B was attributed to the oxidization of their benzotropolone rings and their covalent bonding with ASBT's cysteine. TC uptake was reduced in the COS-7 cells expressing recombinant ASBT whose cysteine residues were mutated to alanine. Finally, the substrate concentration-dependent TC uptake assay showed that TFs competitively inhibited TC uptake.

Membrane lipids and transporter function

Transport proteins are essential for cells in allowing the exchange of substances between cells and their environment across the lipid bilayer forming a tight barrier. Membrane lipids modulate the function of transmembrane proteins such as transporters in two ways: Lipids are tightly and specifically bound to transport proteins and in addition they modulate from the bulk of the lipid bilayer the function of transport proteins. This overview summarizes currently available information at the ultrastructural level on lipids tightly bound to transport proteins and the impact of altered bulk membrane lipid composition. Human diseases leading to altered lipid homeostasis will lead to altered membrane lipid composition, which in turn affect the function of transporter proteins.

[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.

Role of n-3 Fatty Acids on Bile Acid Metabolism and Transport in Dyslipidemia: A Review

Dietary n-3 fatty acids, especially of marine origin, eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3), have always been lauded for their profound effects on regulating the risk factors for major metabolic disorders. Yet, their consumption rate is poor compared to n-6 fatty acids [linoleic acid (18:2n-6)], which are predominantly consumed. Hence, the skewed n-6 to n-3 fatty acid ratio may have a bearing on the risk factors of various diseases, including dyslipidemia. Dyslipidemia and other lifestyle diseases associated with it, such as diabetes, obesity, hypertension, are a growing concern in both developed and developing countries. A common strategy for addressing dyslipidemia involves bile acid (BA) sequestration, to interrupt the enterohepatic circulation of BA, resulting in the modulation of lipid absorption in the intestine, thereby normalizing the levels of circulating lipids. The BA homeostasis is under the tight control of hepatic and enteric BA transporters. Many investigations have reported the effects of dietary constituents, including certain fatty acids on the reabsorption and transport of BA. However, a critical review of the effects of n-3 fatty acids on BA metabolism and transport is not available. The present review attempts to explore certain unmapped facets of the n-3 fatty acids on BA metabolism and transport in dyslipidemia, and their interplay with biological processes involving lipid rafts and gut microbiome.

Bile acid transporter-mediated oral drug delivery

Bile acids are synthesized in the liver, stored in the gallbladder, and secreted into the duodenum at meals. Apical sodium-dependent bile acid transporter (ASBT), an ileal Na⁺-dependent transporter, plays the leading role of bile acid absorption into enterocytes, where bile acids are delivered to basolateral side by ileal bile acid binding protein (IBABP) and then released by organic solute transporter OSTα/β. The absorbed bile acids are delivered to the liver via portal vein. In this process called "enterohepatic recycling", only 5% of the bile acid pool (~3 g in human) is excreted in feces, indicating the large recycling capacity and high transport efficacy of ASBT-mediated absorption. Therefore, bile acid transporter-mediated oral drug delivery has been regarded as a feasible and potential strategy to improve the oral bioavailability. This review introduces the key factors in enterohepatic recycling, especially the mechanism of bile acid uptake by ASBT, and the development of bile acid-based oral drug delivery for ASBT-targeting, including bile acid-based prodrugs, bile acid/drug electrostatic complexation and bile acid-containing nanocarriers. Furthermore, the specific transport pathways of bile acid in enterocytes are described and the recent finding of lymphatic delivery of bile acid-containing nanocarriers is discussed.

S-acylation modulates the function of the apical sodium-dependent bile acid transporter in human cells

The ileal apical sodium-dependent bile acid transporter (ASBT) is crucial for the enterohepatic circulation of bile acids. ASBT function is rapidly regulated by several posttranslational modifications. One reversible posttranslational modification is S-acylation, involving the covalent attachment of fatty acids to cysteine residues in proteins. However, whether S-acylation affects ASBT function and membrane expression has not been determined. Using the acyl resin-assisted capture method, we found that the majority of ASBT (∼80%) was S-acylated in ileal brush border membrane vesicles from human organ donors, as well as in HEK293 cells stably transfected with ASBT (2BT cells). Metabolic labeling with alkyne-palmitic acid (100 μm for 15 h) also showed that ASBT is S-acylated in 2BT cells. Incubation with the acyltransferase inhibitor 2-bromopalmitate (25 μm for 15 h) significantly reduced ASBT S-acylation, function, and levels on the plasma membrane. Treatment of 2BT cells with saturated palmitic acid (100 μm for 15 h) increased ASBT function, whereas treatment with unsaturated oleic acid significantly reduced ASBT function. Metabolic labeling with alkyne-oleic acid (100 μm for 15 h) revealed that oleic acid attaches to ASBT, suggesting that unsaturated fatty acids may decrease ASBT's function via a direct covalent interaction with ASBT. We also identified Cys-314 as a potential S-acylation site. In conclusion, these results provide evidence that S-acylation is involved in the modulation of ASBT function. These findings underscore the potential for unsaturated fatty acids to reduce ASBT function, which may be useful in disorders in which bile acid toxicity is implicated.

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.

A novel bioluminescence-based method to investigate uptake of bile acids in living cells

Bile acid transporters, including the ileal apical sodium-dependent bile acid transporter (ASBT) and the hepatic sodium-taurocholate cotransporting polypeptide (NTCP), are crucial for the enterohepatic circulation of bile acids. Our objective was to develop a method for measuring bile acid transporter activity in real time to precisely evaluate rapid changes in their function. We designed a reporter system relying on a novel probe: cholic acid attached to luciferin via a disulfide-containing, self-immolating linker (CA-SS-Luc). Incubation of human embryonic kidney-293 cells coexpressing luciferase and ASBT with different concentrations of CA-SS-Luc (0.01-1 μM) resulted in bioluminescence with an intensity that was concentration- and time-dependent. The bioluminescence measured during incubation with 1 μM CA-SS-Luc was dependent on the levels of ASBT or NTCP expressed in the cells. Coincubation of CA-SS-Luc with natural bile acids enhanced the bioluminescence in a concentration-dependent manner with kinetic parameters for ASBT similar to those previously reported using conventional methods. These findings suggest that this method faithfully assesses ASBT function. Further, incubation with tyrosine phosphatase inhibitor III (PTPIII) led to significantly increased bioluminescence in cells expressing ASBT, consistent with previous studies showing an increase in ASBT function by PTPIII. We then investigated CA-SS-Luc in isolated mouse intestinal epithelial cells. Ileal enterocytes displayed significantly higher luminescence compared with jejunal enterocytes, indicating a transport process mediated by ileal ASBT. In conclusion, we have developed a novel method to monitor the activity of bile acid transporters in real time that has potential applications both for in vitro and in vivo studies. NEW & NOTEWORTHY This article reports the development of a real-time method for measuring the uptake of bile acids using a bioluminescent bile acid-based probe. This method has been validated for measuring uptake via the apical sodium-dependent bile acid transporter and the sodium-taurocholate cotransporting polypeptide in cell culture and ex vivo intestinal models.

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

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

Chenodeoxycholic acid requires activation of EGFR, EPAC, and Ca2+ to stimulate CFTR-dependent Cl- secretion in human colonic T84 cells

Bile acids are known to initiate intricate signaling events in a variety of tissues, primarily in the liver and gastrointestinal tract. Of the known bile acids, only the 7α-dihydroxy species, deoxycholic acid and chenodeoxycholic acid (CDCA), and their conjugates, activate processes that stimulate epithelial Cl^- secretion. We have previously published that CDCA acts in a rapid manner to stimulate colonic ion secretion via protein kinase A (PKA)-mediated activation of the dominant Cl^- channel, the cystic fibrosis transmembrane conductance regulator (CFTR) (Ao M, Sarathy J, Domingue J, Alrefai WA, and Rao MC. Am J Physiol Cell Physiol 305: C447-C456, 2013); however, PKA signaling did not account for the entire CDCA response. Here we show that in human colonic T84 cells, CDCA's induction of CFTR activity, measured as changes in short-circuit current (I_sc), is dependent on epidermal growth factor receptor (EGFR) activation and does not involve the bile acid receptors TGR5 or farnesoid X receptor. CDCA activation of Cl^- secretion does not require Src, mitogen-activated protein kinases, or phosphoinositide 3-kinase downstream of EGFR but does require an increase in cytosolic Ca²⁺ In addition to PKA signaling, we found that the CDCA response requires the novel involvement of the exchange protein directly activated by cAMP (EPAC). EPAC is a known hub for cAMP and Ca²⁺ cross talk. Downstream of EPAC, CDCA activates Rap2, and changes in free cytosolic Ca²⁺ were dependent on both EPAC and EGFR activation. This study establishes the complexity of CDCA signaling in the colonic epithelium and shows the contribution of EGFR, EPAC, and Ca²⁺ in CDCA-induced activation of CFTR-dependent Cl^- secretion.

Mechanisms of DRA recycling in intestinal epithelial cells: effect of enteropathogenic E. coli

Enteropathogenic Escherichia coli (EPEC) is a food-borne pathogen that causes infantile diarrhea worldwide. EPEC decreases the activity and surface expression of the key intestinal Cl(-)/HCO3(-) exchanger SLC26A3 [downregulated in adenoma (DRA)], contributing to the pathophysiology of early diarrhea. Little is known about the mechanisms governing membrane recycling of DRA. In the current study, Caco-2 cells were used to investigate DRA trafficking under basal conditions and in response to EPEC. Apical Cl(-)/HCO3(-) exchange activity was measured as DIDS-sensitive (125)I(-) uptake. Cell surface biotinylation was performed to assess DRA endocytosis and exocytosis. Inhibition of clathrin-mediated endocytosis by chlorpromazine (60 μM) increased apical Cl(-)/HCO3(-) exchange activity. Dynasore, a dynamin inhibitor, also increased function and surface levels of DRA via decreased endocytosis. Perturbation of microtubules by nocodazole revealed that intact microtubules are essential for basal exocytic (but not endocytic) DRA recycling. Mice treated with colchicine showed a decrease in DRA surface levels as visualized by confocal microscopy. In response to EPEC infection, DRA surface expression was reduced partly via an increase in DRA endocytosis and a decrease in exocytosis. These effects were dependent on the EPEC virulence genes espG1 and espG2. Intriguingly, the EPEC-induced decrease in DRA function was unaltered in the presence of dynasore, suggesting a clathrin-independent internalization of surface DRA. In conclusion, these studies establish the role of clathrin-mediated endocytosis and microtubules in the basal surface expression of DRA and demonstrate that the EPEC-mediated decrease in DRA function and apical expression in Caco-2 cells involves decreased exocytosis.

N-glycosylation is essential for ileal ASBT function and protection against proteases

The bile acid transporter ASBT is a glycoprotein responsible for active absorption of bile acids. Inhibiting ASBT function and bile acid absorption is an attractive approach to lower plasma cholesterol and improve glucose imbalance in diabetic patients. Deglycosylation of ASBT was shown to decrease its function. However, the exact roles of N-glycosylation of ASBT, and how it affects its function, is not known. Current studies investigated the roles of N-glycosylation in ASBT protein stability and protection against proteases utilizing HEK-293 cells stably transfected with ASBT-V5 fusion protein. ASBT-V5 protein was detected as two bands with molecular mass of ~41 and ~35 kDa. Inhibition of glycosylation by tunicamycin significantly decreased ASBT activity and shifted ASBT bands to ~30 kDa, representing a deglycosylated protein. Treatment of total cellular lysates with PNGase F or Endo H glycosidases showed that the upper 41-kDa band represents a fully mature N-acetylglucosamine-rich glycoprotein and the lower 35-kDa band represents a mannose-rich core glycoprotein. Studies with the glycosylation deficient ASBT mutant (N10Q) showed that the N-glycosylation is not essential for ASBT targeting to plasma membrane. However, mature glycosylation significantly increased the half-life and protected ASBT protein from digestion with trypsin. Incubating the cells with high glucose (25 mM) for 48 h increased mature glycosylated ASBT along with an increase in its function. These results unravel novel roles for N-glycosylation of ASBT and suggest that high levels of glucose alter the composition of the glycan and may contribute to the increase in ASBT function in diabetes mellitus.

HEK-293 cells expressing the cystic fibrosis transmembrane conductance regulator (CFTR): a model for studying regulation of Cl- transport

The Human Embryonic Kidney 293 cell line (HEK‐293) readily lends itself to genetic manipulation and is a common tool for biologists to overexpress proteins of interest and study their function and molecular regulation. Although these cells have some limitations, such as an inability to form resistive monolayers necessary for studying transepithelial ion transport, they are nevertheless valuable in studying individual epithelial ion transporters. We report the use of HEK‐293 cells to study the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. While HEK‐293 cells endogenously express mRNA for the Cl⁻ channels, ClC‐2 and TMEM16A, they neither express CFTR mRNA nor protein. Therefore, we stably transfected HEK‐293 cells with EGFP‐CFTR (HEK‐CFTR) and demonstrated CFTR function by measuring forskolin‐stimulated iodide efflux. This efflux was inhibited by CFTR_inh172, and the protein kinase A inhibitor H89, but not by Ca²⁺ chelation. In contrast to intestinal epithelia, forskolin stimulation does not increase surface CFTR expression and does not require intact microtubules in HEK‐CFTR. To investigate the role of an endogenous Gα_S‐coupled receptor, we examined the bile acid receptor, TGR5. Although HEK‐CFTR cells express TGR5, the potent TGR5 agonist lithocholic acid (LCA; 5–500 μmol/L) did not activate CFTR. Furthermore, forskolin, but not LCA, increased [cAMP]_i in HEK‐CFTR suggesting that endogenous TGR5 may not be functionally linked to Gα_S. However, LCA did increase [Ca²⁺]_i and interestingly, abolished forskolin‐stimulated iodide efflux. Thus, we propose that the stable HEK‐CFTR cell line is a useful model to study the multiple signaling pathways that regulate CFTR.

In this study, we characterize a HEK‐293 cell line, stably transfected with EGFP‐CFTR (HEK‐CFTR). We examined its regulation by endogenously expressed signaling pathways, in particular the cAMP and the GαS‐coupled bile acid receptor, TGR5, signaling pathways.

Resveratrol promotes degradation of the human bile acid transporter ASBT (SLC10A2)

The sodium/bile acid co-transporter ASBT [apical sodium-dependent bile acid transporter; SLC10A2 (solute carrier family 10 member 2)] plays a key role in the enterohepatic recycling of the bile acids and indirectly contributes to cholesterol homoeostasis. ASBT inhibitors reportedly lower plasma triglyceride levels and increase HDL (high-density lipoprotein) cholesterol levels. RSV (resveratrol), a major constituent of red wine, is known to lower LDL (low-density lipoprotein) cholesterol levels, but its mechanism of action is still unclear. In the present study, we investigated the possible involvement of ASBT in RSV-mediated cholesterol-lowering effects. We demonstrate that RSV inhibits ASBT protein expression and function via a SIRT1 (sirtuin 1)-independent mechanism. The effect was specific to ASBT since other transporters involved in cholesterol homoeostasis, NTCP (SLC10A1), OSTα (SLC51A) and ABCG1 (ATP-binding cassette G1), remained unaffected. ASBT inhibition by RSV was reversed by proteasome inhibitors (MG-132 and lactacystin) and the ubiquitin inhibitor LDN57444, suggesting involvement of the ubiquitin-proteasome pathway. Immunoprecipitation revealed high levels of ubiquitinated ASBT after RSV treatment. Phosphorylation at Ser335 and Thr339 was shown previously to play a role in proteosomal degradation of rat ASBT. However, mutation at corresponding residues in rat ASBT revealed that phosphorylation does not contribute to RSV-mediated degradation of ASBT. Combined, our data indicate that RSV promotes ASBT degradation via the ubiquitin-proteasome pathway without requiring phosphorylation. We conclude that regulation of ASBT expression by RSV may have clinical relevance with regard to the observed cholesterol-lowering effects of RSV.

Herb-drug interactions: challenges and opportunities for improved predictions

Supported by a usage history that predates written records and the perception that "natural" ensures safety, herbal products have increasingly been incorporated into Western health care. Consumers often self-administer these products concomitantly with conventional medications without informing their health care provider(s). Such herb-drug combinations can produce untoward effects when the herbal product perturbs the activity of drug metabolizing enzymes and/or transporters. Despite increasing recognition of these types of herb-drug interactions, a standard system for interaction prediction and evaluation is nonexistent. Consequently, the mechanisms underlying herb-drug interactions remain an understudied area of pharmacotherapy. Evaluation of herbal product interaction liability is challenging due to variability in herbal product composition, uncertainty of the causative constituents, and often scant knowledge of causative constituent pharmacokinetics. These limitations are confounded further by the varying perspectives concerning herbal product regulation. Systematic evaluation of herbal product drug interaction liability, as is routine for new drugs under development, necessitates identifying individual constituents from herbal products and characterizing the interaction potential of such constituents. Integration of this information into in silico models that estimate the pharmacokinetics of individual constituents should facilitate prospective identification of herb-drug interactions. These concepts are highlighted with the exemplar herbal products milk thistle and resveratrol. Implementation of this methodology should help provide definitive information to both consumers and clinicians about the risk of adding herbal products to conventional pharmacotherapeutic regimens.

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.

Recent advances in small bowel diseases: Part II

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

Enteropathogenic Escherichia coli inhibits ileal sodium-dependent bile acid transporter ASBT

Apical sodium-dependent bile acid transporter (ASBT) is responsible for the absorption of bile acids from the intestine. A decrease in ASBT function and expression has been implicated in diarrhea associated with intestinal inflammation. Whether infection with pathogenic microorganisms such as the enteropathogenic Escherichia coli (EPEC) affect ASBT activity is not known. EPEC is a food-borne enteric pathogen that translocates bacterial effector molecules via type three secretion system (TTSS) into host cells and is a major cause of infantile diarrhea. We investigated the effects of EPEC infection on ileal ASBT function utilizing human intestinal Caco2 cells and HEK-293 cells stably transfected with ASBT-V5 fusion protein (2BT cells). ASBT activity was significantly inhibited following 60 min infection with EPEC but not with nonpathogenic E. coli. Mutations in bacterial escN, espA, espB, and espD, the genes encoding for the elements of bacterial TTSS, ablated EPEC inhibitory effect on ASBT function. Furthermore, mutation in the bacterial BFP gene encoding for bundle-forming pili abrogated the inhibition of ASBT by EPEC, indicating the essential role for bacterial aggregation and the early attachment. The inhibition by EPEC was associated with a significant decrease in the V(max) of the transporter and a reduction in the level of ASBT on the plasma membrane. The inhibition of ASBT by EPEC was blocked in the presence of protein tyrosine phosphatase inhibitors. Our studies provide novel evidence for the alterations in the activity of ASBT by EPEC infection and suggest a possible effect for EPEC in influencing intestinal bile acid homeostasis.

Hu antigen R and tristetraprolin: counter-regulators of rat apical sodium-dependent bile acid transporter by way of effects on messenger RNA stability

The apical sodium-dependent bile acid transporter (ASBT, SLC10A2) mediates intestinal, renal, and cholangiocyte bile acid reclamation. Transcriptional regulation of ASBT is well described, whereas information on posttranscriptional regulation is limited. Prior studies suggested that ontogeny of ASBT is controlled in part by changes in messenger RNA (mRNA) stability. We studied the role that Hu antigen R (HuR) and tristetraprolin (TTP) play in regulating the expression of mRNA that contains the 3' untranslated region (UTR) of rat ASBT. The 3'UTR was incorporated into an SV-40 driven luciferase reporter (rASBT3-luciferase) for rapid screening of regulatory effects. Silencing HuR reduced luciferase reporter activity, whereas silencing TTP enhanced luciferase activity. Conversely, overexpression of HuR enhanced rASBT3-luciferase reporter activity. The same 3'UTR fragments of rat ASBT were incorporated into a beta-globin coding mRNA construct for analysis of mRNA stability (rASBT3-βglobin). mRNA half-life was progressively shortened by the incorporation of increasing sized fragments of the 3'UTR. Silencing HuR shortened the half-life of rASBT3-βglobin containing 0.3 kb of the rat ASBT 3'UTR. Gel shift assays revealed binding of HuR and TTP to rat ASBT 3'UTR. Endogenously expressed human ASBT mRNA half-lives and steady-state protein levels in Caco-2 cells were repressed when HuR was silenced but was enhanced when TTP was silenced. Developmental changes in HuR and TTP protein abundance correlated with previously characterized ontogenic changes in rat ileal and renal ASBT expression.

CONCLUSION

These studies not only show that ASBT expression is controlled at the level of mRNA stability by way of its 3'UTR, but also identify HuR and TTP as two key transacting factors that are involved in exerting counterregulatory effects on ASBT mRNA stability.

Folate malabsorption is associated with down-regulation of folate transporter expression and function at colon basolateral membrane in rats

Folates, an essential component (important B vitamin) in the human diet, are involved in many metabolic pathways, mainly in carbon transfer reactions such as purine and pyrimidine biosynthesis and amino acid interconversions. Deficiency of this micronutrient leads to the disruption of folate-dependent metabolic pathways that lead to the development of clinical abnormalities ranging from anaemia to growth retardation. Folate deficiency due to alcohol ingestion is quite common, primarily due to malabsorption. The present study dealt with the mechanistic insights of folate malabsorption in colonic basolateral membrane (BLM). Wistar rats (n 12) were fed 1 g/kg body weight per d ethanol (20 %) solution orally for 3 months and folate transport was studied in the isolated colonic BLM. The folate exit across colon BLM shows characteristics of carrier-mediated process with the major involvement of reduced folate carrier (RFC). The chronic ethanol ingestion decreased the uptake by decreasing the affinity by 46 % (P < 0·01) and the number of transport molecules by 43 % (P < 0·001) at the colon BLM. The decreased uptake was associated with down-regulation of proton-coupled folate transporter (PCFT) and RFC expression at mRNA and protein levels. The extent of decrease was 44 % (P < 0·01) and 24 % (P < 0·05) for PCFT and 23 % (P < 0·01) and 57 % (P < 0·01) for RFC at mRNA and protein levels, respectively. Moreover, folate transporters were associated with lipid rafts (LR) of colon BLM, and chronic alcoholism decreased the association of these transporters with LR.

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.

Stimulation of apical Cl⁻/HCO₃⁻(OH⁻) exchanger, SLC26A3 by neuropeptide Y is lipid raft dependent

Neuropeptide Y (NPY), an important proabsorptive hormone of the gastrointestinal tract has been shown to inhibit chloride secretion and stimulate NaCl absorption. However, mechanisms underlying the proabsorptive effects of NPY are not fully understood. The present studies were designed to examine the direct effects of NPY on apical Cl⁻/HCO₃⁻(OH⁻) exchange activity and the underlying mechanisms involved utilizing Caco2 cells. Our results showed that NPY (100 nM, 30 min) significantly increased Cl⁻/HCO₃⁻(OH⁻) exchange activity (∼2-fold). Selective NPY/Y1 or Y2 receptor agonists mimicked the effects of NPY. NPY-mediated stimulation of Cl⁻/HCO₃⁻(OH⁻) exchange activity involved the ERK1/2 MAP kinase-dependent pathway. Cell surface biotinylation studies showed that NPY does not alter DRA (apical Cl⁻/HCO₃⁻(OH⁻) exchanger) surface expression, ruling out the involvement of membrane trafficking events. Interestingly, DRA was found to be predominantly expressed in the detergent-insoluble (DI) and low-density fractions (LDF) of human colonic apical membrane vesicles (AMVs) representing lipid rafts. Depletion of membrane cholesterol by methyl-β-cyclodextrin (MβCD, 10 mM, 1 h) remarkably decreased DRA expression in the DI fractions. Similar results were obtained in Triton-X 100-treated Caco2 plasma membranes. DRA association with lipid rafts in the DI and LDF fractions of Caco2 cells was significantly enhanced (∼45%) by NPY compared with control. MβCD significantly decreased Cl⁻/HCO₃⁻(OH⁻) exchange activity in Caco2 cells as measured by DIDS- or niflumic acid-sensitive ³⁶Cl⁻ uptake (∼50%). Our results demonstrate that NPY modulates Cl⁻/HCO₃⁻(OH⁻) exchange activity by enhancing the association of DRA with lipid rafts, thereby resulting in an increase in Cl⁻/HCO₃⁻(OH⁻) exchange activity. Our findings suggest that the alteration in the association of DRA with lipid rafts may contribute to the proabsorptive effects of NPY in the human intestine.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Green tea catechin EGCG inhibits ileal apical sodium bile acid transporter ASBT

Green tea catechins exhibit hypocholesterolemic effects probably via their inhibitory effects on intestinal bile acid absorption. Ileal apical sodium-dependent bile acid transporter (ASBT) is responsible for reabsorption of bile acids. The present studies were, therefore, designed to investigate the modulation of ASBT function and membrane expression by green tea catechins in human embryonic kidney HEK-293 cells stably transfected with ASBT-V5 fusion protein and intestinal Caco-2 monolayers. Our data showed that ASBT activity was significantly decreased by (-)-epigallocatechin-3-gallate (EGCG) but not other green tea catechins. Inhibition of PKC, phosphatidylinositol 3-kinase, and MAPK-dependent pathways failed to block the reduction in ASBT activity by EGCG. Kinetics studies showed a significant decrease in the V(max) of the transporter, whereas total ASBT content on the plasma membrane was unaltered by EGCG. Concomitant with the decrease in ASBT function, EGCG significantly reduced ASBT pool in the detergent-insoluble fraction, while increasing its presence in the detergent-soluble fraction of plasma membrane. Furthermore, EGCG decreased the association of ASBT with floating lipid raft fractions of cellular membrane on Optiprep density gradient. In conclusion, our data demonstrate a novel role of lipid rafts in the modulation of ASBT function by the dietary component EGCG, which may underlie the hypocholesterolemic effects of green tea.

Modulation of ileal apical Na+-dependent bile acid transporter ASBT by protein kinase C

Ileal apical Na(+)-dependent bile acid transporter (ASBT) is responsible for reabsorbing the majority of bile acids from the intestinal lumen. Rapid adaptation of ASBT function in response to physiological and pathophysiological stimuli is essential for the maintenance of bile acid homeostasis. However, not much is known about molecular mechanisms responsible for acute posttranscriptional regulation of ileal ASBT. The protein kinase C (PKC)-dependent pathway represents a major cell signaling mechanism influencing intestinal epithelial functions. The present studies were, therefore, undertaken to investigate ASBT regulation in intestinal Caco-2 monolayers by the well-known PKC activator phorbol 12-myristate 13-acetate (PMA). Our results showed that Na(+)-dependent [(3)H]taurocholic acid uptake in Caco-2 cells was significantly inhibited in response to 2 h incubation with 100 nM PMA compared with incubation with 4alpha-PMA (inactive form). The inhibitory effect of PMA was blocked in the presence of 5 microM bisindolylmaleimide I (PKC inhibitor) but not 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (Ca(2+) chelator) or LY-294002 (phosphatidylinositol 3-kinase inhibitor). PMA inhibition of ASBT function was also abrogated in the presence of myristoylated PKCzeta pseudosubstrate peptide, indicating involvement of the atypical PKCzeta isoform. The inhibition by PMA was associated with a significant decrease in the maximal velocity of the transporter and a reduction in ASBT plasma membrane content, suggesting a modulation by vesicular recycling. Our novel findings demonstrate a posttranscriptional modulation of ileal ASBT function and membrane expression by phorbol ester via a PKCzeta-dependent pathway.

Bile acid transporters

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

Bile acid transporters

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

Cholesterol feeding prevents hepatic accumulation of bile acids in cholic acid-fed farnesoid X receptor (FXR)-null mice: FXR-independent suppression of intestinal bile acid absorption

Cholic acid (CA) feeding of farnesoid X receptor (Fxr)-null mice results in markedly elevated hepatic bile acid levels and liver injury. In contrast, Fxr-null mice fed cholesterol plus CA (CA+Chol) do not exhibit liver injury, and hepatic bile acid levels and bile acid pool size are reduced 51 and 40%, respectively, compared with CA-treated Fxr-null mice. These decreases were not observed in wild-type mice. Despite a reduced bile acid pool size, hepatic Cyp7a1 mRNA expression was increased in Fxr-null mice fed the CA+Chol diet, and biliary bile acid output was not changed. Analysis of other potential protective mechanisms revealed significant decreases in portal blood bile acid concentrations and a reduced ileal bile acid absorption capacity, as estimated using an in situ loop method. Fecal bile acid excretion was also increased in Fxr-null mice fed the CA+Chol versus CA diet. The decreased ileal bile acid absorption correlated with decreased ileal apical sodium-dependent bile salt transporter (ASBT) protein expression in brush-border membranes. These results suggest a critical role for ileal bile acid absorption in regulation of hepatic bile acid levels in Fxr-null mice fed CA+Chol. Furthermore, experiments with Fxr-null mice suggest that cholesterol feeding can down-regulate ASBT expression through a pathway independent of FXR.