Structural basis of bile salt extrusion and small-molecule inhibition in human BSEP

Enterohepatic circulation-inspired nano-platform for less-hepatotoxicity hepatocellular carcinoma therapy

Hepatocellular carcinoma (HCC) has a poor prognosis because it is often diagnosed after clinical deterioration and lacks effective therapies. The advent of tumor-targeting therapeutics provided a promise in the landscape of advanced HCC but it is still ongoing due to the indistinguishable receptor expression and receptor abundance between hepatocytes and HCC cells. Herein, a GSH-responsive prodrug, CA-PEG-ss-PTX, was synthesized with cholic acid (CA), paclitaxel (PTX) and polyethylene glycol (PEG) and further applied to physically encapsulate PTX, forming PTX/CA-PEG-ss-PTX (PTX/CPSP). PTX/CPSP gained enhanced liver accumulation via CA-mediated active targeting. After internalization in HCC cells, PTX/CPSP could rapidly disassociate and release PTX in response to the high-level GSH for tumor killing. However, it could remain intact in hepatocytes. Furthermore, CA-modification significantly increased the biliary excretion of PTX/CPSP and performed a "fast in ∼ fast out" drug delivery pattern in hepatocytes, thereby reducing the toxicity caused by excessive drug accumulation. Finally, PTX/CPSP displayed superior anti-HCC efficacy with tolerable toxicity. It is worth noting that PTX/CPSP achieved satisfied PTX loading efficiency (more than 30 %) by both chemical synthesis and physical encapsulation. In summary, with all parts being clinically available or endogenous, PTX/CPSP is considered a clinical potential HCC treatment strategy.

Repression of bile salt efflux pump expression by tri-ortho-cresyl phosphate in cultured human hepatic cells

Tri-ortho-cresyl phosphate (TOCP) is an environmental toxic pollutant, belonging to the chemical class of organophosphorus flame retardants and repressing hepatic membrane drug transporter expression. The present study investigated whether the liver canalicular bile salt efflux pump (BSEP) may also be targeted by TOCP. TOCP used at a non-cytotoxic concentration of 10 μM for 48 h was demonstrated to decrease BSEP mRNA expression in cultured hepatic HepaRG cells (by a 4.4-fold factor) and primary human hepatocytes (by a 2.5-fold factor). This effect was concentration-dependent (IC50 = 0.8 μM) and was associated with a significant reduction of canalicular taurocholate secretion in HepaRG cells. It was not impaired by TOCP metabolism inhibitors. TOCP also potently antagonized farnesoid-X-receptor (FXR) mediated-BSEP up-regulation. The specific FXR antagonist DY268 decreased constitutive BSEP expression in HepaRG cells, as TOCP, suggesting a major implication of FXR antagonism in TOCP effects towards BSEP. The TOCP-mediated BSEP repression was finally predicted to potentially occur in vivo in response to a neurotoxic dose or to acute or chronic safe doses of TOCP. Taken together, these data demonstrate that the major bile salt transporter BSEP is a target for TOCP, which may support deleterious hepatotoxic effects of this chemical.

Membrane Proteins in Nanodiscs: Methods and Applications

Membrane proteins, a principal class of drug targets, play indispensable roles in various biological processes and are closely associated with essential life functions. Their study, however, is complicated by their low solubility in aqueous environments and distinctive structural characteristics, necessitating a suitable native-like environment for molecular analysis. Nanodisc technology has revolutionized this field, providing biochemists with a powerful tool to stabilize membrane proteins and significantly enhance their research possibilities. This review outlines the substantial advancements in nanodisc methodologies and applications from 2018 to 2024. We cover the development of various nanodisc models, as well as structural and functional studies of membrane proteins that utilize nanodiscs, highlighting their medical applications.

A structural and mechanistic model for BSEP dysfunction in PFIC2 cholestatic disease

BSEP (ABCB11) transports bile salts across the canalicular membrane of hepatocytes, where they are incorporated into bile. Biallelic mutations in BSEP can cause Progressive Familial Intrahepatic Cholestasis Type 2 (PFIC2), a rare pediatric disease characterized by hepatic bile acid accumulation leading to hepatotoxicity and, ultimately, liver failure. The most frequently occurring PFIC2 disease-causing mutations are missense mutations, which often display a phenotype with decreased protein expression and impaired maturation and trafficking to the canalicular membrane. To characterize the mutational effects on protein thermodynamic stability, we carried out biophysical characterization of 13 distinct PFIC2-associated variants using in-cell thermal shift (CETSA) measurements. These experiments reveal a cluster of residues localized to the NBD2-ICL2 interface, which exhibit severe destabilization relative to wild-type BSEP. A high-resolution (2.8 Å) cryo-EM structure provides a framework for rationalizing the CETSA results, revealing a novel, NBD2-localized mechanism through which the most severe missense patient mutations drive cholestatic disease. These findings suggest potential strategies for identifying mechanism-based small molecule correctors to address BSEP trafficking defects and advance novel therapies for PFIC2 and other cholestatic diseases.

Human liver cell-based assays for the prediction of hepatic bile acid efflux transporter inhibition by drugs

INTRODUCTION

Drug-mediated inhibition of bile salt efflux transporters may cause liver injury. In vitro prediction of drug effects toward canalicular and/or sinusoidal efflux of bile salts from human hepatocytes is therefore a major issue, which can be addressed using liver cell-based assays.

AREA COVERED

This review, based on a thorough literature search in the scientific databases PubMed and Web of Science, provides key information about hepatic transporters implicated in bile salt efflux, the human liver cell models available for investigating functional inhibition of bile salt efflux, the different methodologies used for this purpose, and the modes of expression of the results. Applications of the assays to drugs are summarized, with special emphasis to the performance values of some assays for predicting hepatotoxicity/cholestatic effects of drugs.

EXPERT OPINION

Human liver cell-based assays for evaluating drug-mediated inhibition of bile acid efflux transporters face various limitations, such as the lack of method standardization and validation, the present poor adaptability to high throughput approaches, and some pitfalls with respect to interpretation of bile acid biliary excretion indexes. Hepatotoxicity of drugs is additionally likely multifactorial, highlighting that inhibition of hepatic bile salt efflux by drugs provides important, but not full, information about potential drug hepatotoxicity.

Membrane-dependent dynamics and dual translocation mechanisms of ABCB4: Insights from molecular dynamics simulations

ABCB4 is an ATP-binding cassette transporter expressed at the canalicular membrane of hepatocytes and responsible for translocating phosphatidylcholine into bile. Despite the recent cryo-EM structures of ABCB4, knowledge about the molecular mechanism of phosphatidylcholine transport remains fragmented. In this study, we used all-atom molecular dynamics simulations to investigate ABCB4 dynamics during its transport cycle, leveraging both symmetric and asymmetric membrane models. Our results demonstrate that membrane composition influences the local conformational dynamics of ABCB4, revealing distinct lipid-binding patterns across different conformers, particularly for cholesterol. We explored the two potential mechanisms for phosphatidylcholine translocation: the canonical ATP-driven alternating access model and the "credit-card swipe" model. Critical residues were identified for phosphatidylcholine binding and transport pathway modulation, supporting the canonical mechanism while also indicating a possible additional pathway. The conformer-specific roles of kinking in transmembrane helices (TMH4 and TMH10) were highlighted as key events in substrate translocation. Overall, ABCB4 may utilize a cooperative transport mechanism, integrating elements of both models to facilitate efficient phosphatidylcholine motion across the membrane. This study provides new insights into the relationship between membrane environment and ABCB4 function, contributing to our understanding of its role in bile physiology and susceptibility to genetic and xenobiotic influences.

Sodium butyrate activates peroxisome proliferator-activated receptor γ to suppress lithogenic diet-induced cholesterol gallstones in mice

Peroxisome proliferator-activated receptor γ (PPAR-γ) is crucial in forming cholesterol stones. Sodium butyrate (NaB), a short-chain fatty acid, shows potential for gallstone treatment by activating PPAR-γ. This study aimed to elucidate the effects of NaB on cholesterol gallstones in mice fed a lithogenic diet (LD). Ezetimibe (5 mg/day) was used as a positive control, and a PPAR-γ antagonist (CW9661, 4 mg/kg/day) was used to investigate PPAR-γ. Body weight, gallstone incidence, lipid concentrations in blood, bile, and liver, liver function evaluation, histological analysis, and cholesterol metabolism-related gene expression were evaluated. NaB and ezetimibe suppressed gallstone formation, serum AST, ALT, and ALP levels, and serum/liver TG and TC. They also reduced bile cholesterol and phospholipids, and liver histological damage. NaB activated PPAR-γ, CYP7A1, ABCA1, and ABCB11 while suppressing ABCG5/G8 gene expression. CW9661 reversed NaB's benefits in LD mice. This study provides scientific evidence that NaB activated PPAR-γ to improve gallstones.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-024-01721-x.

NLRP3 Inflammasome Activation Is Involved in Geniposide-Induced Hepatotoxicity

Background: Geniposide, a prominent iridoid glycoside derived from Gardenia jasminoides J. Ellis, has garnered attention due to its association with hepatotoxicity despite its well-documented pharmacological efficacy in preclinical and clinical contexts. The NOD-like receptor protein 3 (NLRP3) inflammasome is implicated in numerous pathological conditions, including drug-induced liver injury. This study aims to explore the involvement of the NLRP3 inflammasome in geniposide-induced liver toxicity. Methods: Rats were administered geniposide for 5 days, concurrently treated with or without glibenclamide (GLY), an in vivo inhibitor of NLRP3. In vitro, HL-7702 cells were exposed to genipin (a metabolite of geniposide via hepatointestinal circulation), with or without GLY supplement. Liver tissue was examined through pathological sections. Aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), γ-glutamyl transpeptidase (γ-GT), and total bilirubin (T-Bil) levels were determined using the enzyme plate method. IL-1β and IL-18 levels in the supernatant and serum were quantified through ELISA. Apoptosis-associated speck-like protein (ASC), NLRP3, caspase-1, pro-IL-1β, and pro-IL-18 mRNA levels in cells or the liver were assessed by RT-PCR. Protein levels of ASC, NLRP3, caspase-1, pro-IL-1β, and pro-IL-18 in cells or the liver were analyzed by Western blot. Results: Rats treated with geniposide displayed notable liver damage characterized by inflammatory infiltration, elevated serum transaminases, and heightened levels of inflammatory factors IL-1β and IL-18. This liver damage was concomitant with NLRP3 inflammasome activation within the liver. Furthermore, genipin induction led to reduced cell viability, increased transaminases in the cell supernatant, and an upsurge in inflammatory factors, resulting in heightened NLRP3 inflammasome expression within the cells. However, GLY effectively curtailed excessive NLRP3 activation, dampened the production of inflammatory factors IL-1β and IL-18, and ameliorated liver damage caused by geniposide. Conclusions: Our findings collectively elucidate that geniposide induces hepatotoxicity by triggering NLRP3 inflammasome signaling. Inhibition of the inflammasome presents a promising novel therapeutic target for mitigating geniposide-induced hepatotoxicity.

Successful strategies for expression and purification of ABC transporters

ATP-binding cassette (ABC) transporters are proteins responsible for active transport of various compounds, from small ions to macromolecules, across membranes. Proteins from this superfamily also pump drugs out of the cell resulting in multidrug resistance. Based on the cellular functions of ABC-transporters they are commonly associated with diseases like cancer and cystic fibrosis. To understand the molecular mechanism of this critical family of integral membrane proteins, structural characterization is a powerful tool which in turn requires successful recombinant production of stable and functional protein in good yields. In this review we have used high resolution structures of ABC transporters as a measure of successful protein production and summarized strategies for prokaryotic and eukaryotic proteins, respectively. In general, Escherichia coli is the most frequently used host for production of prokaryotic ABC transporters while human embryonic kidney 293 (HEK293) cells are the preferred host system for eukaryotic proteins. Independent of origin, at least two-steps of purification were required after solubilization in the most used detergent DDM. The purification tag was frequently cleaved off before structural characterization using cryogenic electron microscopy, or crystallization and X-ray analysis for prokaryotic proteins.

A comparative study on the toxic effects of lead pollution and nanoplastic-lead mixed pollution on red drum and their detoxification strategies

Nanoplastics and heavy metals pose various adverse effects on marine organisms. However, the combined toxicity of nanoplastics and lead pollution to marine fish is not fully understood. This study investigates the toxic effects and detoxification strategies of lead pollution (p07) compared to nanoplastic-lead mixed pollution (m07) in red drum during exposure and recovery phases. Under m07 pollution, the maximum lead content in muscle was 22.61 mg/kg, which was significantly higher than the 15.82 mg/kg observed under p07 pollution. This finding demonstrated that nanoplastics significantly enhance lead accumulation, leading to more severe toxic effects on red drum. Histological analyses revealed that lipid droplets in the liver and epithelial lifting in the gills were the primary lesion types. During the exposure periods, red drum primarily detoxified lead through cellular renewal and the removal of damaged proteins under p07 pollution. Conversely, under m07 pollution, detoxification relied on cellular senescence, apoptosis, endocytosis, and the removal of damaged proteins. In the recovery phases, red drum predominantly recovered through cell proliferation and antioxidant responses under p07 pollution. Under m07 pollution, the focus shifted to functional protein synthesis, apoptosis, endocytosis, and lipid metabolism. This study offers valuable insights into the monitoring and management of combined environmental pollution.

ATP-Binding Cassette and Solute Carrier Transporters: Understanding Their Mechanisms and Drug Modulation Through Structural and Modeling Approaches

The ATP-binding cassette (ABC) and solute carrier (SLC) transporters play pivotal roles in cellular transport mechanisms, influencing a wide range of physiological processes and impacting various medical conditions. Recent advancements in structural biology and computational modeling have provided significant insights into their function and regulation. This review provides an overview of the current knowledge of human ABC and SLC transporters, emphasizing their structural and functional relationships, transport mechanisms, and the contribution of computational approaches to their understanding. Current challenges and promising future research and methodological directions are also discussed.

A 3D spheroid model of quadruple cell co-culture with improved liver functions for hepatotoxicity prediction

Drug-induced liver injury (DILI) is one of the major concerns during drug development. Wide acceptance of the 3 R principles and the innovation of in-vitro techniques have introduced various novel model options, among which the three-dimensional (3D) cell spheroid cultures have shown a promising prospect in DILI prediction. The present study developed a 3D quadruple cell co-culture liver spheroid model for DILI prediction via self-assembly. Induction by phorbol 12-myristate 13-acetate at the concentration of 15.42 ng/mL for 48 hours with a following 24-hour rest period was used for THP-1 cell differentiation, resulting in credible macrophagic phenotypes. HepG2 cells, PUMC-HUVEC-T1 cells, THP-1-originated macrophages, and human hepatic stellate cells were selected as the components, which exhibited adaptability in the designated spheroid culture conditions. Following establishment, the characterization demonstrated the competence of the model in long-term stability reflected by the maintenance of morphology, viability, cellular integration, and cell-cell junctions for at least six days, as well as the reliable liver-specific functions including superior albumin and urea secretion, improved drug metabolic enzyme expression and CYP3A4 activity, and the expression of MRP2, BSEP, and P-GP accompanied by the bile acid efflux transport function. In the comparative testing using 22 DILI-positive and 5 DILI-negative compounds among the novel 3D co-culture model, 3D HepG2 spheroids, and 2D HepG2 monolayers, the 3D culture method significantly enhanced the model sensitivity to compound cytotoxicity compared to the 2D form. The novel co-culture liver spheroid model exhibited higher overall predictive power with margin of safety as the classifying tool. In addition, the non-parenchymal cell components could amplify the toxicity of isoniazid in the 3D model, suggesting their potential mediating role in immune-mediated toxicity. The proof-of-concept experiments demonstrated the capability of the model in replicating drug-induced lipid dysregulation, bile acid efflux inhibition, and α-SMA upregulation, which are the key features of liver steatosis and phospholipidosis, cholestasis, and fibrosis, respectively. Overall, the novel 3D quadruple cell co-culture spheroid model is a reliable and readily available option for DILI prediction.

The noncanonical nucleotide binding site 1 of the bile salt export pump is optimized for proper function of the transporter

The bile salt export pump (ABCB11/BSEP) is a hepatocyte plasma membrane-resident protein translocating bile salts into bile canaliculi. The sequence alignment of the four full-length transporters of the ABCB subfamily (ABCB1, ABCB4, ABCB5 and ABCB11) indicates that the NBD-NBD contact interface of ABCB11 differs from that of other members in only four residues. Notably, these are all located in the noncanonical nucleotide binding site 1 (NBS1). Substitution of all four deviant residues with canonical ones (quadruple mutant) significantly decreased the transport activity of the protein. In this study, we mutated two deviant residues in the signature sequence to generate a double mutant (R1221G/E1223Q). Furthermore, a triple mutant (E502S/R1221G/E1223Q) was generated, in which the deviant residues of the signature sequence and Q-loop were mutated concurrently to canonical residues. The double and triple mutants showed 80% and 60%, respectively, of the activity of wild-type BSEP. As expected, an increasing number of mutations gradually impair transport as an intricate network of interactions within the ABC proteins ensures proper functioning.