Kinetic characterization of bile salt transport by human NTCP (SLC10A1)

The role of peroxisome proliferator-activated receptors in the regulation of bile acid metabolism

Bile acids are synthesized from cholesterol in the liver. Dysregulation of bile acid homeostasis, characterized by excessive accumulation in the liver, gallbladder and blood, can lead to hepatocellular damage and the development of cholestatic liver disease. Nuclear receptors play a crucial role in the control of bile acid metabolism by efficiently regulating bile acid synthesis and transport in the liver. Among these receptors, peroxisome proliferator-activated receptor (PPAR), a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily, controls the expression of genes involved in adipogenesis, lipid metabolism, inflammation and glucose homeostasis and has emerged as a potential therapeutic target for the treatment of the metabolic syndrome in the past two decades. Emerging evidence suggests that PPAR activation holds promise as a therapeutic target for cholestatic liver disease, as it affects both bile acid production and transport. This review provides a comprehensive overview of recent advances in elucidating the role of PPAR in the regulation of bile acid metabolism, highlighting the current position of PPAR agonists in the treatment of primary biliary cholangitis. By summarizing the specific regulatory effects of PPAR on bile acids, this review contributes to the exploration of novel therapeutic strategies for cholestatic liver diseases.

Bile acids-mediated intracellular cholesterol transport promotes intestinal cholesterol absorption and NPC1L1 recycling

Niemann-Pick C1-like 1 (NPC1L1) is essential for intestinal cholesterol absorption. Together with the cholesterol-rich and Flotillin-positive membrane microdomain, NPC1L1 is internalized via clathrin-mediated endocytosis and transported to endocytic recycling compartment (ERC). When ERC cholesterol level decreases, NPC1L1 interacts with LIMA1 and moves back to plasma membrane. However, how cholesterol leaves ERC is unknown. Here, we find that, in male mice, intracellular bile acids facilitate cholesterol transport to other organelles, such as endoplasmic reticulum, in a non-micellar fashion. When cholesterol level in ERC is decreased by bile acids, the NPC1L1 carboxyl terminus that previously interacts with the cholesterol-rich membranes via the A1272LAL residues dissociates from membrane, exposing the Q1277KR motif for LIMA1 recruitment. Then NPC1L1 moves back to plasma membrane. This study demonstrates an intracellular cholesterol transport function of bile acids and explains how the substantial amount of cholesterol in NPC1L1-positive compartments is unloaded in enterocytes during cholesterol absorption.

Role of Hepatocyte Transporters in Drug-Induced Liver Injury (DILI)-In Vitro Testing

Bile acids and bile salts (BA/BS) are substrates of both influx and efflux transporters on hepatocytes. Canalicular efflux transporters, such as BSEP and MRP2, are crucial for the removal of BA/BS to the bile. Basolateral influx transporters, such as NTCP, OATP1B1/1B3, and OSTα/β, cooperate with canalicular transporters in the transcellular vectorial flux of BA/BS from the sinusoids to the bile. The blockage of canalicular transporters not only impairs the bile flow but also causes the intracellular accumulation of BA/BS in hepatocytes that contributes to, or even triggers, liver injury. In the case of BA/BS overload, the efflux of these toxic substances back to the blood via MRP3, MRP4, and OST α/β is considered a relief function. FXR, a key regulator of defense against BA/BS toxicity suppresses de novo bile acid synthesis and bile acid uptake, and promotes bile acid removal via increased efflux. In drug development, the early testing of the inhibition of these transporters, BSEP in particular, is important to flag compounds that could potentially inflict drug-induced liver injury (DILI). In vitro test systems for efflux transporters employ membrane vesicles, whereas those for influx transporters employ whole cells. Additional in vitro pharmaceutical testing panels usually include cellular toxicity tests using hepatocytes, as well as assessments of the mitochondrial toxicity and accumulation of reactive oxygen species (ROS). Primary hepatocytes are the cells of choice for toxicity testing, with HepaRG cells emerging as an alternative. Inhibition of the FXR function is also included in some testing panels. The molecular weight and hydrophobicity of the drug, as well as the steady-state total plasma levels, may positively correlate with the DILI potential. Depending on the phase of drug development, the physicochemical properties, dosing, and cut-off values of BSEP IC₅₀ ≤ 25-50 µM or total C_ss,plasma/BSEP IC₅₀ ≥ 0.1 may be an indication for further testing to minimize the risk of DILI liability.

Bile Acids: Physiological Activity and Perspectives of Using in Clinical and Laboratory Diagnostics

Bile acids play a significant role in the digestion of nutrients. In addition, bile acids perform a signaling function through their blood-circulating fraction. They regulate the activity of nuclear and membrane receptors, located in many tissues. The gut microbiota is an important factor influencing the effects of bile acids via enzymatic modification. Depending on the rate of healthy and pathogenic microbiota, a number of bile acids may support lipid and glucose homeostasis as well as shift to more toxic compounds participating in many pathological conditions. Thus, bile acids can be possible biomarkers of human pathology. However, the chemical structure of bile acids is similar and their analysis requires sensitive and specific methods of analysis. In this review, we provide information on the chemical structure and the biosynthesis of bile acids, their regulation, and their physiological role. In addition, the review describes the involvement of bile acids in various diseases of the digestive system, the approaches and challenges in the analysis of bile acids, and the prospects of their use in omics technologies.

Sodium+/taurocholate cotransporting polypeptide as target therapy for liver fibrosis

OBJECTIVE

Sodium⁺/ taurocholate cotransporting polypeptide (NTCP) is a membrane transporter affecting the enterohepatic circulation of bile acids (BAs). We aimed to evaluate NTCP's roles in humans and animal models of liver fibrosis (LF).

DESIGN

Primary hepatic stellate cells (pHSCs) isolated from livers biopsies of patients with LF with different fibrosis grading were stained for NTCP. NTCP gene silencing, taurocholic acid (TCA), obeticholic acid (OCA), epigallocatechin gallate (EGCG) and HA-100 dihydrochloride (HA-100) were used as tools to modulate NTCP expression on human HSC line (LX2). BA trafficking/uptake were assessed extracellularly (LX2 culture medium) and intracellularly following treatment with/without NTCP neutralizing antibody. LF models of C57/BL6 mice of carbon tetrachloride (CCl₄) and leptin-deficient (Ob/Ob) fed with high-fat diet (Ob/Ob ^HFD ) were evaluated for pHSCs-NTCP expressions, metabolic and LF profiles following intraperitoneal injections of NTCP neutralizing antibody.

RESULTS

pHSCs from F3/F4-scored patients of LF exhibit threefold increased NTCP expressions compared with F0-scored patients (p<0.0001). Sorted-activated HSCs (LX2^αSMA+) showed high expressions of NTCP and high TCA uptake in vitro and triggered a further increase in their activations. This phenomenon was inhibited with NTCP small interfering RNA and the NTCP neutralizing antibody. Sorted LX2^NTCP+ (high alpha smooth muscle actin (αSMA)/high NTCP) cells showed high phosphorylated pathways of AKT/mTOR and protein kinase C (PKC) accompanied with a decrease in farnesoid X receptor expression. Moreover, LX2^NTCP+ cells treated with EGCG, OCA and PKC inhibitor HA-100 significantly decreased NTCP and αSMA. NTCP neutralizing antibody inhibited NTCP (less TCA uptake); it attenuated LF in both CCl₄ and Ob/Ob ^HFD animal models with ameliorated metabolic profile.

CONCLUSION

NTCP expression is linearly correlated with fibrosis severity. Modulated BA trafficking could be an important step in LF pathogenesis. Antagonising BA uptake may suggest a therapeutic strategy for preventing disease progression.

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

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

Comprehensive Analysis of Long Non-coding RNA and mRNA Transcriptomes Related to Hypoxia Adaptation in Tibetan Sheep

Tibetan sheep have lived on the Qinghai-Tibet Plateau for a long time, and after long-term natural selection, they have shown stable genetic adaptability to high-altitude environments. However, little is known about the molecular mechanisms of the long non-coding (lnc)RNAs involved in the adaptation of Tibetan sheep to hypoxia. Here, we collected lung tissues from high-altitude Tibetan sheep and low-altitude Hu sheep for RNA sequencing to study the regulatory mechanisms of the lncRNAs and mRNAs in the adaptation of Tibetan sheep to hypoxia. We identified 254 differentially expressed lncRNAs and 1,502 differentially expressed mRNAs. We found 20 pairs of cis-regulatory relationships between 15 differentially expressed lncRNAs and 14 protein-coding genes and two pairs of trans-regulatory relationships between two differentially expressed lncRNAs and two protein-coding genes. These differentially expressed mRNAs and lncRNA target genes were mainly enriched in pathways related to lipid metabolism and immune function. Interaction network analysis showed that 17 differentially expressed lncRNAs and 15 differentially expressed mRNAs had an interactive relationship. Additionally, we used six differentially expressed lncRNAs and mRNAs to verify the accuracy of the sequencing data via qRT-PCR. Our results provide a comprehensive overview of the expression patterns of the lncRNAs and mRNAs involved in the adaptation of Tibetan sheep to hypoxia, laying a foundation for further analysis of the adaptations of plateau animals.

Function and Expression of Bile Salt Export Pump in Suspension Human Hepatocytes

The mechanistic understanding of bile salt disposition is not well established in suspension human hepatocytes (SHH) because of the limited information on the expression and function of bile salt export protein (BSEP) in this system. We investigated the transport function of BSEP in SHH using a method involving in situ biosynthesis of bile salts from their precursor bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA). Our data indicated that glycine- and taurine-conjugated CA and CDCA were generated efficiently and transported out of hepatocytes in a concentration- and time-dependent manner. We also observed that the membrane protein abundance of BSEP was similar between SHH and sandwich-cultured human hepatocytes. Furthermore, known cholestatic agents significantly inhibited G-CA and G-CDCA efflux in SHH. Interestingly, cyclosporine A, troglitazone, itraconazole, loratadine, and lovastatin inhibited G-CA efflux more potently than G-CDCA efflux (3- to 5-fold). Because of these significant differential effects on G-CA and G-CDCA efflux inhibition, we determined the IC₅₀ values of troglitazone for G-CA (9.9 µM) and for G-CDCA (43.1 µM) efflux. The observed discrepancy in the IC₅₀ was attributed to the fact that troglitazone also inhibits organic anion transporting polypeptides and Na⁺/taurocholate cotransporting polypeptide in addition to BSEP. The hepatocyte uptake study suggested that both active uptake and passive diffusion contribute to the liver uptake of CA, whereas CDCA primarily undergoes passive diffusion into the liver. In summary, these data demonstrated the expression and function of BSEP and its major role in transport of bile salts in cryopreserved SHH. SIGNIFICANCE STATEMENT: BSEP transport function and protein abundance was evident in SHH in the present study. The membrane abundance of BSEP protein was similar between SHH and sandwich-cultured human hepatocytes. The study also illustrated the major role of BSEP relative to basolateral MRP3 and MRP4 in transport of bile salts in SHH. Understanding of BSEP function in SHH may bolster the utility of this platform in mechanistic understanding of bile salt disposition and potentially in the assessment of drugs for BSEP inhibition.

Assessment of Statin Interactions With the Human NTCP Transporter Using a Novel Fluorescence Assay

Sodium taurocholate cotransporting polypeptide (NTCP), which is highly expressed in the sinusoidal membrane of hepatocytes, maintains bile acid homeostasis and participates in the hepatic disposition of a variety of endogenous substances as well as xenobiotics. Manifested by the involvement of organic anion-transporting polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3) in the hepatic uptake of statin drugs, sinusoidal membrane transporters play an important role in the pharmacokinetics and pharmacodynamics of these agents. It has been speculated that NTCP may function as an alternative pathway for statin hepatic uptake, complementary to OATP1B1 and OATP1B3. In the current study, we produced stable NTCP-expressing human embryonic kidney 293 (HEK293) cells and developed a fluorescence-based assay using flow cytometry for measuring NTCP transport with chenodeoxycholyl-(Nε-7-nitrobenz-2-oxa-1,3-diazole)-lysine (CDCA-NBD) as the substrate. NTCP-mediated CDCA-NBD transport was time-dependent and exhibited typical Michaelis-Menten kinetics, with a K _m of 6.12 µM. Compounds known to interact with NTCP, including chenodeoxycholic acid and taurocholic acid, displayed concentration-dependent inhibition of NTCP-mediated CDCA-NBD transport. We report here a systematic evaluation of the interaction between statins and the NTCP transporter. Utilizing this system, several statins were either found to inhibit NTCP-dependent transport or act as substrates. We find a good correlation between the reported lipophilicity of statins and their ability to inhibit NTCP. The objective was to develop a higher-throughput system to evaluate potential inhibitors such as the statins. The in vitro assays using CDCA-NBD as fluorescent substrate are convenient, rapid, and have utility in screening drug candidates for potential drug-NTCP interactions.

Prediction of Drug-Induced Hyperbilirubinemia by In Vitro Testing

Bilirubin, the end product of heme catabolism, is produced continuously in the body and may reach toxic levels if accumulates in the serum and tissues; therefore, a highly efficient mechanism evolved for its disposition. Normally, unconjugated bilirubin enters hepatocytes through the uptake transporters organic anion transporting polypeptide (OATP) 1B1 and 1B3, undergoes glucuronidation by the Phase II enzyme UDP glucuronosyltransferase 1A1 (UGT1A1), and conjugated forms are excreted into the bile by the canalicular export pump multidrug resistance protein 2 (MRP2). Any remaining conjugated bilirubin is transported back to the blood by MRP3 and passed on for uptake and excretion by downstream hepatocytes or the kidney. The bile salt export pump BSEP as the main motor of bile flow is indirectly involved in bilirubin disposition. Genetic mutations and xenobiotics that interfere with this machinery may impede bilirubin disposition and cause hyperbilirubinemia. Several pharmaceutical compounds are known to cause hyperbilirubinemia via inhibition of OATP1Bs, UGT1A1, or BSEP. Herein we briefly review the in vitro prediction methods that serve to identify drugs with a potential to induce hyperbilirubinemia. In vitro assays can be deployed early in drug development and may help to minimize late-stage attrition. Based on current evidence, drugs that behave as mono- or multispecific inhibitors of OATP1B1, UGT1A1, and BSEP in vitro are at risk of causing clinically significant hyperbilirubinemia. By integrating inhibition data from in vitro assays, drug serum concentrations, and clinical reports of hyperbilirubinemia, predictor cut-off values have been established and are provisionally suggested in this review. Further validation of in vitro readouts to clinical outcomes is expected to enhance the predictive power of these assays.

Phase I study of PF‐04895162, a Kv7 channel opener, reveals unexpected hepatotoxicity in healthy subjects, but not rats or monkeys: clinical evidence of disrupted bile acid homeostasis

During a randomized Phase 1 clinical trial the drug candidate, PF‐04895162 (ICA‐105665), caused transaminase elevations (≥grade 1) in six of eight healthy subjects treated at 300 mg twice daily for 2‐weeks (NCT01691274). This was unexpected since studies in rats (<6 months) and cynomolgus monkeys (<9 months) treated up to 100 mg/kg/day did not identify the liver as a target organ. Mechanistic studies showed PF‐04895162 had low cytotoxic potential in human hepatocytes, but inhibited liver mitochondrial function and bile salt export protein (BSEP) transport. Clinical relevance of these postulated mechanisms of liver injury was explored in three treated subjects that consented to analysis of residual pharmacokinetic plasma samples. Compared to a nonresponder, two subjects with transaminase elevations displayed higher levels of miRNA122 and total/conjugated bile acid species, whereas one demonstrated impaired postprandial clearance of systemic bile acids. Elevated taurine and glycine conjugated to unconjugated bile acid ratios were observed in two subjects, one before the onset of elevated transaminases. Based on the affinity of conjugated bile acid species for transport by BSEP, the profile of plasma conjugated/unconjugated bile acid species was consistent with inhibition of BSEP. These data collectively suggest that the human liver injury by PF‐04895162 was due to alterations in bile acid handling driven by dual BSEP/mitochondrial inhibition, two important risk factors associated with drug‐induced liver injury in humans. Alterations in systemic bile acid composition were more important than total bile acids in the manifestation of clinical liver injury and may be a very early biomarker of BSEP inhibition.

Weighted gene coexpression network analysis identifies specific transcriptional modules and hub genes related to intramuscular fat traits in chicken breast muscle

Intramuscular fat (IMF) traits are important factors that influence meat quality. However, the molecular regulatory mechanisms that underlie this trait in chickens are still poorly understood at the gene coexpression level. Here, we performed a weighted gene coexpression network analysis between IMF traits and transcriptome profile in breast muscle in the Chinese domestic Gushi chicken breed at 6, 14, 22, and 30 weeks. A total of 26 coexpressed gene modules were identified. Six modules, which included the dark gray, purple, cyan, pink, light cyan, and blue modules, showed a significant positive correlation (P < 0.05) with IMF traits. The strongest correlation was observed between the dark gray module and IMF content (r = 0.85; P = 4e-04) and between the blue module and different fatty acid content (r = 0.87~0.91; P = 5e-05~2e-04). Enrichment analysis showed that the enrichment of biological processes, such as fatty acid metabolic process, fat cell differentiation, acylglycerol metabolic process, and glycerolipid metabolism were significantly different in the six modules. In addition, the 32, 24, 4, 7, 6, and 25 hub genes were identified from the blue, pink, light cyan, cyan, dark gray, and purple modules, respectively. These hub genes are involved in multiple links to fatty acid metabolism, phospholipid metabolism, cholesterol metabolism, diverse cellular behaviors, and cell events. These results provide novel insights into the molecular regulatory mechanisms for IMF-related traits in chicken and may also help to uncover the formation mechanism for excellent meat quality traits in local breeds of Chinese chicken.