The feline bile salt export pump: a structural and functional comparison with canine and human Bsep/BSEP

Human multidrug resistance protein 4 (MRP4) is a cellular efflux transporter for paracetamol glutathione and cysteine conjugates

Paracetamol (acetaminophen, APAP) overdose is a leading cause of acute drug-induced liver failure. APAP hepatotoxicity is mediated by the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is inactivated by conjugation with glutathione (GSH) to APAP-GSH, which is further converted into its cysteine derivative APAP-CYS. Before necrosis of hepatocytes occurs, APAP-CYS is measurable in plasma of the affected patient and it has been proposed as an early biomarker of acetaminophen toxicity. APAP-GSH and APAP-CYS can be extruded by hepatocytes, but the transporters involved are unknown. In this study we examined whether ATP-binding cassette (ABC) transporters play a role in the cellular efflux of APAP, APAP-GSH, and APAP-CYS. The ABC transport proteins P-gp/ABCB1, BSEP/ABCB11, BCRP/ABCG2, and MRP/ABCC1-5 were overexpressed in HEK293 cells and membrane vesicles were produced. Whereas P-gp, BSEP, MRP3, MRP5, and BCRP did not transport any of the compounds, uptake of APAP-GSH was found for MRP1, MRP2 and MRP4. APAP-CYS appeared to be a substrate of MRP4 and none of the ABC proteins transported APAP. The results suggest that the NAPQI metabolite APAP-CYS can be excreted into plasma by MRP4, where it could be a useful biomarker for APAP exposure and toxicity. Characterization of the cellular efflux of APAP-CYS is important for its development as a biomarker, because plasma concentrations might be influenced by drug-transporter interactions and upregulation of MRP4.

Effect of a Common Genetic Variant (p.V444A) in the Bile Salt Export Pump on the Inhibition of Bile Acid Transport by Cholestatic Medications

The bile salt export pump (BSEP) is the primary canalicular transporter responsible for the secretion of bile acids from hepatocytes into bile canaliculi, and inhibition of this transporter has been associated with drug-induced liver injury (DILI). A common variant (rs2287622; p.V444A) in the gene encoding BSEP has been associated with an increased risk of cholestatic DILI. Although p.444V BSEP (reference) and p.444A BSEP (variant) do not differ in their transport kinetics of taurocholic acid (TCA), transport of the more abundant glycocholic acid (GCA) has not been investigated. Importantly, differences in the susceptibility of p.444V and p.444A BSEP to inhibition by drugs causing cholestatic DILI have not been investigated. To address these issues, the transport kinetics of GCA were evaluated by incubating membrane vesicles expressing either p.444V or p.444A BSEP with GCA over a range of concentrations (1, 10, 25, 50, and 100 μM). The abilities of commonly used cholestatic medications to inhibit the transport of TCA and GCA by the reference and variant proteins were compared. Resulting data indicated that GCA transport kinetics for reference and variant BSEP followed Michaelis-Menten kinetics and were not statistically different [ V_max values of 1132 ± 246 and 959 ± 256 pmol min^-1 (mg of protein)^-1, respectively, and K_m values of 32.7 ± 18.2 and 45.7 ± 25.5 μM, respectively]. There were no statistically significant differences between the reference and variant BSEP in the inhibition of TCA or GCA transport by the cholestatic drugs tested. In conclusion, differential inhibition of TCA or GCA transport cannot account for an association between the variant BSEP and the risk for cholestatic DILI due to the drugs tested.

Uremic solutes modulate hepatic bile acid handling and induce mitochondrial toxicity

Chronic kidney disease (CKD) is accompanied by accumulating levels of uremic solutes in the circulation. Changes in the size and composition of the bile acid pool have also been observed. We investigated via which mechanisms uremic solutes may interfere with hepatocyte function and thus contribute to altered bile acid handling. We studied interference on the level of bile acid synthesis by cytochrome P450 7A1 (CYP7A1), explored effects on hepatic bile acid transporters, and investigated effects on mitochondrial function. In HEK293 cells overexpressing bile salt transporters, we observed that p-cresyl sulfate inhibited Na⁺-taurocholate cotransporting polypeptide (NTCP)-mediated uptake of taurocholic acid (TCA), whereas organic anion-transporting polypeptide 1B1 (OATP1B1)-mediated TCA uptake was increased. Assays in transporter-overexpressing membrane vesicles revealed that kynurenic acid inhibited TCA transport via the bile salt efflux pump (BSEP), whereas p-cresyl glucuronide and hippuric acid increased TCA efflux via multidrug resistance-associated protein 3 (MRP3). Moreover, indoxyl sulfate decreased mRNA expression of NTCP, OATP1B3 and CYP7A1 in primary human hepatocytes. Transport studies confirmed a decreased TCA uptake in indoxyl sulfate-exposed hepatocytes. Decreased hepatocyte viability was found for all seven uremic solutes tested, whereas five out of seven also decreased intracellular ATP levels and mitochondrial membrane potential. In conclusion, uremic solutes affect hepatic bile acid transport and mitochondrial function. This can contribute to the altered bile acid homeostasis observed in CKD patients.

Feline biliary tree and gallbladder disease: Aetiology, diagnosis and treatment

Practical relevance Diseases of the biliary tree and gallbladder are more common in cats than diseases of the liver parenchyma. The parenchyma is usually affected secondarily to systemic illnesses, while the biliary system is the prime target for infectious agents (eg, bacteria and flukes) and non-infectious conditions (eg, neoplasia and cysts). Clinical approaches Cats with biliary disease are evaluated because of common feline clinical signs such as anorexia, nausea, vomiting and lethargy. Icterus may or may not be obvious. Biopsies for histological evaluation, and bile aspirates for culture and cytological evaluation are helpful diagnostically. Antibiotics and immunosuppressive drugs have been used successfully. Hepatosupportive drugs may help in liquefying thick bile and protecting hepatic tissue from damage. Ultrasound is a noninvasive diagnostic tool that may help in identifying dilated bile ducts, liver cysts and choleliths. It is also used to guide percutaneous bile aspiration. Audience This review, written for all veterinarians who treat cats, describes the various conditions that can affect the feline biliary tree and gallbladder. Treatment options are discussed, and brief summaries provided of surgical techniques and diagnostic approaches. Evidence base The veterinary literature pertaining to feline biliary disease is comprehensively reviewed. When appropriate, data on dogs and humans has been included to provide background information. Based on the available literature, more research into feline biliary diseases is needed.

Genetic and Proteomic characterization of Bile Salt Export Pump (BSEP) in Snake Liver

Snake gallbladder, a traditional Chinese medicine, has been believed in various Asian countries to improve visual acuity and alleviate rheumatism. Bile acids, a major component of the gallbladder, are toxic to the liver and kidney in humans and animals due to its detergent effects, while also exhibiting therapeutic effects due to an increase in the gallbladder contractions of muscle strips in patients with cholesterol gallstones. Secretion of bile acids in human and mammals depends on the bile salt export pump (BSEP), a liver-specific adenosine triphosphate (ATP)-binding cassette transporter encoded by ABCB11. However, the presence of BSEP in snakes has not been thoroughly explored. Here we confirm the existence of BSEP and its coding DNA sequence in snakes on both the proteomic and genetic level. This work provides information on the snake ABCB11 sequence and helps further potential genetic manipulation to affect bile salt metabolism. Our study provides the foundation for research on bile acid production from snakes by using modern genetic and proteomic methodologies.

Recent advances in the exploration of the bile salt export pump (BSEP/ABCB11) function

INTRODUCTION

The bile salt export pump (BSEP/ABCB11), residing in the apical membrane of hepatocyte, mediates the secretion of bile salts into the bile. A range of human diseases is associated with the malfunction of BSEP, including fatal hereditary liver disorders and mild cholestatic conditions. Manifestation of these diseases primarily depends on the mutation type; however, other factors such as hormonal changes and drug interactions can also trigger or influence the related diseases.

AREAS COVERED

Here, we summarize the recent knowledge on BSEP by covering its transport properties, cellular localization, regulation and major mutations/polymorphisms, as well as the hereditary and acquired diseases associated with BSEP dysfunction. We discuss the different model expression systems employed to understand the function of the BSEP variants, their drug interactions and the contemporary therapeutic interventions.

EXPERT OPINION

The limitations of the available model expression systems for BSEP result in controversial conclusions, and obstruct our deeper insight into BSEP deficiencies and BSEP-related drug interactions. The knowledge originating from different methodologies, such as clinical studies, molecular genetics, as well as in vitro and in silico modeling, should be integrated and harmonized. Increasing availability of robust molecular biological tools and our better understanding of the mechanism of BSEP deficiencies should make the personalized, mutation-based therapeutic interventions more attainable.

Atovaquone and quinine anti-malarials inhibit ATP binding cassette transporter activity

Background

Therapeutic blood plasma concentrations of anti-malarial drugs are essential for successful treatment. Pharmacokinetics of pharmaceutical compounds are dependent of adsorption, distribution, metabolism, and excretion. ATP binding cassette (ABC) transport proteins are particularly involved in drug deposition, as they are located at membranes of many uptake and excretory organs and at protective barriers, where they export endogenous and xenobiotic compounds, including pharmaceuticals. In this study, a panel of well-established anti-malarial drugs which may affect drug plasma concentrations was tested for interactions with human ABC transport proteins.

Methods

The interaction of chloroquine, quinine, artemisinin, mefloquine, lumefantrine, atovaquone, dihydroartemisinin and proguanil, with transport activity of P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), bile salt export pump (BSEP) and multidrug resistance-associated proteins (MRP) 1–4 were analysed. The effect of the anti-malarials on the ATP-dependent uptake of radio-labelled substrates was measured in membrane vesicles isolated from HEK293 cells overexpressing the ABC transport proteins.

Results

A strong and previously undescribed inhibition of BCRP-mediated transport by atovaquone with a 50% inhibitory concentration (IC₅₀) of 0.23 μM (95% CI 0.17-0.29 μM) and inhibition of P-gp-mediated transport by quinine with an IC₅₀ of 6.8 μM (95% CI 5.9-7.8 μM) was observed. Furthermore, chloroquine and mefloquine were found to significantly inhibit P-gp-mediated transport. BCRP transport activity was significantly inhibited by all anti-malarials tested, whereas BSEP-mediated transport was not inhibited by any of the compounds. Both MRP1- and MRP3-mediated transport were significantly inhibited by mefloquine.

Conclusions

Atovaquone and quinine significantly inhibit BCRP- and P-gp- mediated transport at concentrations within the clinically relevant prophylactic and therapeutic range. Co-administration of these established anti-malarials with drugs that are BCRP or P-gp substrates may potentially lead to drug-drug interactions.

In silico identification and in vitro validation of potential cholestatic compounds through 3D ligand-based pharmacophore modeling of BSEP inhibitors

Drug-induced cholestasis is a frequently observed side effect of drugs and is often caused by an unexpected interaction with the bile salt export pump (BSEP/ABCB11). BSEP is the key membrane transporter responsible for the transport of bile acids from hepatocytes into bile. Here, we developed a pharmacophore model that describes the molecular features of compounds associated with BSEP inhibitory activity. To generate input and validation data sets, in vitro experiments with membrane vesicles overexpressing human BSEP were used to assess the effect of compounds (50 μM) on BSEP-mediated (3)H-taurocholic acid transport. The model contains two hydrogen bond acceptor/anionic features, two hydrogen bond acceptor vector features, four hydrophobic/aromatic features, and exclusion volumes. The pharmacophore was validated against a set of 59 compounds, including registered drugs. The model recognized 9 out of 12 inhibitors (75%), which could not be identified based on general parameters, such as molecular weight or SlogP, alone. Finally, the model was used to screen a virtual compound database. A number of compounds found via virtual screening were tested and displayed statistically significant BSEP inhibition, ranging from 13 ± 1% to 67 ± 7% of control (P < 0.05). In conclusion, we developed and validated a pharmacophore model that describes molecular features found in BSEP inhibitors. The model may be used as an in silico screening tool to identify potentially harmful drug candidates at an early stage in drug development.