Bile Salt Export Pump Activity Research Articles

High-Throughput Assessment of Bile Salt Export Pump Inhibition Using RapidFire-MS and DESI-MS.

The bile salt export pump (BSEP) assay is widely used to evaluate the potential for drug-induced liver injury (DILI) early in the drug discovery process. While traditional liquid chromatography-mass spectrometry (LC-MS)-based approaches have been utilized for BSEP activity testing, they have intrinsic limitations in either throughput or the requirement for sample preparation and are difficult to scale up in order to screen drug candidates. Here we demonstrate the use of two different high-throughput MS methods based on solid-phase extraction (SPE) and desorption electrospray ionization (DESI) for high-throughput BSEP activity assessment in a label-free manner, with minimal needs for sample workup, at sampling rates of ∼11 and ∼5.5 s/sample, respectively. Both approaches were validated, compared, and successfully applied to the evaluation of 96 drug candidates for the inhibition of taurocholic acid (TCA) transport using BSEP vesicles.

Identification of bile salt export pump inhibitors using machine learning: Predictive safety from an industry perspective

Bile salt export pump (BSEP) is a transporter that moves bile salts from hepatocytes into bile canaliculi. BSEP inhibition can result in the toxic accumulation of bile salts in the liver, which has been identified as a risk factor of drug-induced liver injury (DILI). Since DILI is a frequent cause of drug withdrawals from the market or failings in drug development, in vitro BSEP activity is measured with the [3H]taurocholate uptake assay and a half-maximal inhibitory concentration (IC50) higher than 30 µM is advised. Herein, a machine learning classification model was developed to accurately detect BSEP inhibitors and help in the prioritization of in vitro testing. Regression models for the numerical prediction of IC50 values were also generated. Classification and regression models for BSEP inhibition have been evaluated on realistic settings, which is critical prior to ML-based decision making in drug discovery programs. This work illustrates how predictive safety can help in early toxicity risk assessment and compound prioritization by leveraging Novartis historical experimental data.

Metformin Disrupts Bile Acid Efflux by Repressing Bile Salt Export Pump Expression.

The bile salt export pump (BSEP), a key player in hepatic bile acid clearance, has been the center of research on drug-induced cholestasis. However, such studies focus primarily on the direct inhibition of BSEP, often overlooking the potential impact of transcriptional repression. This work aims to explore the disruption of bile acid efflux caused by drug-induced BSEP repression. BSEP activity was analyzed in human primary hepatocytes (HPH) using a traditional biliary-clearance experiment and a modified efflux assay, which includes a 72-h pretreatment prior to efflux measurement. Relative mRNA and protein expressions were examined by RT-PCR and Western blotting, respectively. Metformin concentration-dependently repressed BSEP expression in HPH. Although metformin did not directly inhibit BSEP activity, longer metformin exposure reduced BSEP transport function in HPH by down-regulating BSEP expression. BSEP repression by metformin was found to be AMP-activated protein kinase-independent. Additional screening of 10 reported cholestatic non-BSEP inhibitors revealed that the anti-cancer drug tamoxifen also markedly repressed BSEP expression and reduced BSEP activity in HPH. Repression of BSEP alone is sufficient to disrupt hepatic bile acid efflux. Metformin and tamoxifen appear to be prototypes of a class of BSEP repressors that may cause drug-induced cholestasis through gene repression instead of direct BSEP inhibition.

Modeling the Effect of BSEP Inhibitors in Lipid Bilayers by Means of All-Atom Molecular Dynamics Simulation

The human bile salt export pump (BSEP) is a membrane protein expressed on the canalicular plasma membrane domain of hepatocytes, which mediates the active transport of unconjugated and conjugated bile salts from liver cells into bile. Genetically inherited defects in BSEP expression or activity causes cholestatic liver injury, and many drugs that cause cholestatic drug-induced liver injury (DILI) in humans have been shown to inhibit BSEP activity in vitro and in vivo, suggesting that this could be one of the mechanisms that initiates human DILI. The relationship between BSEP inhibition and molecular physicochemical properties has been previously investigated, identifying calculated lipophilicity and molecular weight to be significantly correlated with the BSEP inhibition. Predictive BSEP classification models, constructed through multiple quantitative structure–activity relationship modeling approaches, exhibit significant anomalies with differences in experimental IC50 values of three orders of magnitude...

Inhibition of Hepatobiliary Transport Activity by the Antibacterial Agent Fusidic Acid: Insights into Factors Contributing to Conjugated Hyperbilirubinemia/Cholestasis.

Conjugated hyperbilirubinemia accompanied by cholestasis is a frequent side effect during chronic treatment with the antimicrobial agent fusidic acid. Previous studies from our laboratory, addressing mechanisms of musculoskeletal toxicity arising from coadministration of fusidic acid with statins, demonstrated the ability of fusidic acid to potently inhibit human organic anion transporting polypeptides OATP1B1 (IC50 = 1.6 μM) and OATP1B3 (IC50 = 2.5 μM), which are responsible for the uptake-limited clearance of statins as well as bilirubin glucuronide conjugates. In the present work, inhibitory effects of fusidic acid were characterized against additional human hepatobiliary transporters [Na+/taurocholate cotransporting polypeptide (NTCP), bile salt export pump (BSEP), and multidrug resistance-associated proteins MRP2 and MRP3] as well as uridine glucuronosyl transferase (UGT1A1), which mediate the disposition of bile acids and bilirubin (and its conjugated metabolites). Fusidic acid demonstrated concentration-dependent inhibition of human NTCP- and BSEP-mediated taurocholic acid transport with IC50 values of 44 and 3.8 μM, respectively. Inhibition of BSEP activity by fusidic acid was also consistent with the potent disruption of cellular biliary flux (AC50 = 11 μM) in the hepatocyte imaging assay technology assay, with minimal impact on other toxicity end points (e.g., cytotoxicity, mitochondrial membrane potential, reactive oxygen species generation, glutathione depletion, etc.). Fusidic acid also inhibited UGT1A1-catalyzed β-estradiol glucuronidation activity in human liver microsomes with an IC50 value of 16 μM. Fusidic acid did not demonstrate any significant inhibition of ATP-dependent LTC4 transport (IC50's > 300 μM) in human MRP2 or MRP3 vesicles. R values, which reflect maximal in vivo inhibition, were estimated from a static mathematical model by taking into consideration the IC50 values generated in the various in vitro assays and clinically efficacious unbound fusidic acid concentrations. The magnitudes of in vivo interaction (R values) resulting from the inhibition of OATP1B1, UGT1A1, NTCP, and BSEP transport were ∼1.9-2.6, 1.1-1.2, 1.0-1.1, and 1.4-1.7, respectively, which are indicative of some degree of inherent toxicity risk, particularly via inhibition of OATP and BSEP. Collectively, these observations indicate that inhibition of human BSEP by fusidic acid could affect bile acid homeostasis, resulting in cholestatic hepatotoxicity in the clinic. Lack of direct inhibitory effects on MRP2 transport by fusidic acid suggests that conjugated hyperbilirubinemia does not arise via interference in MRP2-mediated biliary disposition of bilirubin glucuronides. Instead, it is possible that elevation in the level of bilirubin conjugates in blood is mediated through inhibition of hepatic OATPs, which are responsible for their reuptake and/or downregulation of MRP2 transporter as a consequence of cholestatic injury.

In vitro model systems to investigate bile salt export pump (BSEP) activity and drug interactions: A review

The bile salt export pump protein (BSEP), expressed on the canalicular membranes of hepatocytes, is primarily responsible for the biliary excretion of bile salts. The inhibition of BSEP transport activity can lead to an increase in intracellular bile salt levels and liver injury. This review discusses the various in vitro assays currently available for assessing the effect of drugs or other chemical entities to modulate BSEP transport activity. BSEP transporter assays use one of the following platforms: Xenopus laevis oocytes; canalicular membrane vesicles (CMV); BSEP-expressed membrane vesicles; cell lines expressing BSEP; sandwich cultured hepatocytes (SCH); and hepatocytes in suspension. Two of these, BSEP-expressed insect membrane vesicles and sandwich cultured hepatocytes, are the most commonly used assays. BSEP membrane vesicles prepared from transfected insect cells are useful for assessing BSEP inhibition or substrate specificity and exploring mechanisms of BSEP-associated genetic diseases. This model can be applied in a high-throughput format for discovery-drug screening. However, experimental results from use of membrane vesicles may lack physiological relevance and the model does not allow for investigation of in situ metabolism in modulation of BSEP activity. Hepatocyte-based assays that use the SCH format provide results that are generally more physiologically relevant than membrane assays. The SCH model is useful in detailed studies of the biliary excretion of drugs and BSEP inhibition, but due to the complexity of SCH preparation, this model is used primarily for determining biliary clearance and BSEP inhibition in a limited number of compounds. The newly developed hepatocyte in suspension assay avoids many of the complexities of the SCH method. The use of pooled cryopreserved hepatocytes in suspension minimizes genetic variance and individual differences in BSEP activity and also provides the opportunity for higher throughput screening and cross-species comparisons.

Two Case Reports of Successful Treatment of Cholestasis With Steroids in Patients With PFIC-2

Mutations in the gene encoding the canalicular bile salt export pump (BSEP) can result in progressive familial intrahepatic cholestasis type 2 (PFIC-2). Treatment options are limited, and PFIC-2 often necessitates liver transplantation. We report on a young woman and a boy who clinically presented with PFIC-2 phenotypes and dramatically improved with steroid treatment. Gene sequencing of ABCB11 encoding for BSEP revealed 2 relevant mutations in both patients. The young woman was compound heterozygous for p.T919del and p.R1235X. At the age of 5 years, partial biliary diversion was performed and rescued liver function but left serum bile salt levels elevated. At age 23 she developed systemic lupus erythematosus. Unexpectedly, steroid therapy normalized serum bile salt levels, with a strong correlation with the steroid dose. She is currently in clinical remission. The boy was compound heterozygous for the ABCB11 mutations c.150+3A>C and p.R832C and presented with intractable pruritus. When he developed colitis, he was treated with steroids. The pruritus completely disappeared and relapsed when steroids were withdrawn. To date, with low-dose budesonide, the boy has been symptom-free for >3 years. In conclusion, the clinical courses suggest that patients with BSEP deficiency and residual BSEP activity may benefit from steroid-based therapy, which represents a new treatment option.

Mitigating the Inhibition of Human Bile Salt Export Pump by Drugs: Opportunities Provided by Physicochemical Property Modulation, In Silico Modeling, and Structural Modification

The human bile salt export pump (BSEP) is a membrane protein expressed on the canalicular plasma membrane domain of hepatocytes, which mediates active transport of unconjugated and conjugated bile salts from liver cells into bile. BSEP activity therefore plays an important role in bile flow. In humans, genetically inherited defects in BSEP expression or activity cause cholestatic liver injury, and many drugs that cause cholestatic drug-induced liver injury (DILI) in humans have been shown to inhibit BSEP activity in vitro and in vivo. These findings suggest that inhibition of BSEP activity by drugs could be one of the mechanisms that initiate human DILI. To gain insight into the chemical features responsible for BSEP inhibition, we have used a recently described in vitro membrane vesicle BSEP inhibition assay to quantify transporter inhibition for a set of 624 compounds. The relationship between BSEP inhibition and molecular physicochemical properties was investigated, and our results show that lipophilicity and molecular size are significantly correlated with BSEP inhibition. This data set was further used to build predictive BSEP classification models through multiple quantitative structure-activity relationship modeling approaches. The highest level of predictive accuracy was provided by a support vector machine model (accuracy = 0.87, κ = 0.74). These analyses highlight the potential value that can be gained by combining computational methods with experimental efforts in early stages of drug discovery projects to minimize the propensity of drug candidates to inhibit BSEP.

Type 2 inositol 1,4,5-trisphosphate receptor modulates bile salt export pump activity in rat hepatocytes

Bile salt secretion is mediated primarily by the bile salt export pump (Bsep), a transporter on the canalicular membrane of the hepatocyte. However, little is known about the short-term regulation of Bsep activity. Ca(2+) regulates targeting and insertion of transporters in many cell systems, and Ca(2+) release near the canalicular membrane is mediated by the type II inositol 1,4,5-trisphosphate receptor (InsP3R2), so we investigated the possible role of InsP3R2 in modulating Bsep activity. The kinetics of Bsep activity were monitored by following secretion of the fluorescent Bsep substrate cholylglycylamido-fluorescein (CGamF) in rat hepatocytes in collagen sandwich culture, an isolated cell system in which structural and functional polarity is preserved. CGamF secretion was nearly eliminated in cells treated with Bsep small interfering RNA (siRNA), demonstrating specificity of this substrate for Bsep. Secretion was also reduced after chelating intracellular calcium, inducing redistribution of InsP3R2 by depleting the cell membrane of cholesterol, or reducing InsP3R function by either knocking down InsP3R2 expression using siRNA or pharmacologic inhibition using xestospongin C. Confocal immunofluorescence showed that InsP3R2 and Bsep are in close proximity in the canalicular region, both in rat liver and in hepatocytes in sandwich culture. However, after knocking down InsP3R2 or inducing its dysfunction with cholesterol depletion, Bsep redistributed intracellularly. Finally, InsP3R2 was lost from the pericanalicular region in animal models of estrogen- and endotoxin-induced cholestasis. These data provide evidence that pericanalicular calcium signaling mediated by InsP3R2 plays an important role in maintaining bile salt secretion through posttranslational regulation of Bsep, and suggest that loss or redistribution of InsP3R2 may contribute to the pathophysiology of intrahepatic cholestasis.

Interference with Bile Salt Export Pump Function Is a Susceptibility Factor for Human Liver Injury in Drug Development

The bile salt export pump (BSEP) is an efflux transporter, driving the elimination of endobiotic and xenobiotic substrates from hepatocytes into the bile. More specifically, it is responsible for the elimination of monovalent, conjugated bile salts, with little or no assistance from other apical transporters. Disruption of BSEP activity through genetic disorders is known to manifest in clinical liver injury such as progressive familial intrahepatic cholestasis type 2. Drug-induced disruption of BSEP is hypothesized to play a role in the development of liver injury for several marketed or withdrawn therapeutics. Unfortunately, preclinical animal models have been poor predictors of the liver injury associated with BSEP interference observed for humans, possibly because of interspecies differences in bile acid composition, differences in hepatobiliary transporter modulation or constitutive expression, as well as other mechanisms. Thus, a BSEP-mediated liver liability may go undetected until the later stages of drug development, such as during clinical trials or even postlicensing. In the absence of a relevant preclinical test system for BSEP-mediated liver injury, the toxicological relevance of available in vitro models to human health rely on the use of benchmark compounds with known clinical outcomes, such as marketed or withdrawn drugs. In this study, membrane vesicles harvested from BSEP-transfected insect cells were used to assess the activity of more than 200 benchmark compounds to thoroughly investigate the relationship between interference with BSEP function and liver injury. The data suggest a relatively strong association between the pharmacological interference with BSEP function and human hepatotoxicity. Although the most accurate translation of risk would incorporate pharmacological potency, pharmacokinetics, clearance mechanisms, tissue distribution, physicochemical properties, indication, and other drug attributes, the additional understanding of a compound's potency for BSEP interference should help to limit or avoid BSEP-related liver liabilities in humans that are not often detected by standard preclinical animal models.

540 Intestinal Subepithelial Myofibroblasts of Ulcerative Colitis Patients Express Increased WNT5A Levels That Enhance Proliferation in an Autocrine Manner

Hepatic bile salt secretion is mediated primarily by the bile salt export pump (BSEP), an ABC transporter on the canalicular membrane of the hepatocyte. Defects in bile salt secretion lead to the profound consequences of intrahepatic cholestasis, sometimes culminating in liver failure. Studies on the short term regulation of BSEP have implicated cAMP and other signaling pathways, but have yielded contradictory results regarding calcium. In order to clarify the role of calcium in BSEP regulation, we examined pericanalicular calcium signaling mediated by the inositol 1,4,5-trisphosphate receptor type 2 (InsP3R2), which is the principal calcium release channel in hepatocytes and is concentrated in the peri-canalicular domain. BSEP and InsP3R2 expression and function were examined in rat liver and in rat hepatocytes in collagen sandwich culture. Using confocal immunofluorescence microscopy, InsP3R2 and BSEP were both found in the region of the canalicular membrane. BSEP activity was examined bymonitoring the kinetics of secretion of CGamF, a fluorescent BSEP substrate, into the sealed canalicular vacuoles of hepatocytes in sandwich culture. CGamF secretion was significantly inhibited by either (1) treating hepatocytes with the calcium chelator BAPTA for 30 minutes; (2) inducing redistribution of InsP3R2 away from the canalicular region by depleting the plasma membrane of cholesterol with 5 mM methyl-beta-cyclodextrin for 30 minutes; (3) knocking down InsP3R2 expression using a specific siRNA; or (4) knocking down BSEP expression using BSEP-specific siRNA as a positive control for the secretion assay. Moreover, knockdown of InsP3R2 significantly decreased bile salt secretion without altering BSEP expression or localization, as confirmed by immunofluorescence microscopy. In separate studies, basal bile salt secretion was reduced in livers from InsP3R2 KO mice, despite maintenance of BSEP localization and expression, as assessed by immunofluorescence microscopy and western blot, respectively. These data provide evidence that pericanalicular calcium signaling mediated by InsP3R2 plays an important role in maintaining bile salt secretion via post-translational regulation of BSEP.We postulate this regulationmay involvemodulating insertion of BSEP into the canalicular membrane .

539 Inositol 1,4,5-Trisphosphate Receptor Type 2 Modulates Bsep Activity in Hepatocytes

Hepatic bile salt secretion is mediated primarily by the bile salt export pump (BSEP), an ABC transporter on the canalicular membrane of the hepatocyte. Defects in bile salt secretion lead to the profound consequences of intrahepatic cholestasis, sometimes culminating in liver failure. Studies on the short term regulation of BSEP have implicated cAMP and other signaling pathways, but have yielded contradictory results regarding calcium. In order to clarify the role of calcium in BSEP regulation, we examined pericanalicular calcium signaling mediated by the inositol 1,4,5-trisphosphate receptor type 2 (InsP3R2), which is the principal calcium release channel in hepatocytes and is concentrated in the peri-canalicular domain. BSEP and InsP3R2 expression and function were examined in rat liver and in rat hepatocytes in collagen sandwich culture. Using confocal immunofluorescence microscopy, InsP3R2 and BSEP were both found in the region of the canalicular membrane. BSEP activity was examined bymonitoring the kinetics of secretion of CGamF, a fluorescent BSEP substrate, into the sealed canalicular vacuoles of hepatocytes in sandwich culture. CGamF secretion was significantly inhibited by either (1) treating hepatocytes with the calcium chelator BAPTA for 30 minutes; (2) inducing redistribution of InsP3R2 away from the canalicular region by depleting the plasma membrane of cholesterol with 5 mM methyl-beta-cyclodextrin for 30 minutes; (3) knocking down InsP3R2 expression using a specific siRNA; or (4) knocking down BSEP expression using BSEP-specific siRNA as a positive control for the secretion assay. Moreover, knockdown of InsP3R2 significantly decreased bile salt secretion without altering BSEP expression or localization, as confirmed by immunofluorescence microscopy. In separate studies, basal bile salt secretion was reduced in livers from InsP3R2 KO mice, despite maintenance of BSEP localization and expression, as assessed by immunofluorescence microscopy and western blot, respectively. These data provide evidence that pericanalicular calcium signaling mediated by InsP3R2 plays an important role in maintaining bile salt secretion via post-translational regulation of BSEP.We postulate this regulationmay involvemodulating insertion of BSEP into the canalicular membrane .