Sodium taurocholate cotransporting polypeptide (SLC10A1) deficiency: conjugated hypercholanemia without a clear clinical phenotype

Reduced spontaneous itch in mouse models of cholestasis

Pruritus is one of the most distressing symptoms in cholestatic patients. Plasma autotaxin (ATX) activity correlates with the severity of pruritus in cholestatic patients, but the pathophysiology is unclear. To study pruritus in mice, we measured scratch activity in cholestatic Atp8b1 mutant mice, a model for Progressive Familial Intrahepatic Cholestasis type 1, and wild type mice (WT) with alpha-naphthylisothiocyanate (ANIT)-induced cholestasis. To induce cholestasis, Atp8b1 mutant mice received a diet containing 0.1% cholic acid (CA) and WT mice were treated with ANIT. In these mice ATX was also overexpressed by transduction with AAV-ATX. Scratch activity was measured using an unbiased, electronic assay. Marked cholestasis was accomplished in both Atp8b1 mutant mice on a CA-supplemented diet and in ANIT-treatment in WT mice, but scratch activity was decreased rather than increased while plasma ATX activity was increased. Plasma ATX activity was further increased up to fivefold with AAV-ATX, but this did not induce scratch activity. In contrast to several reports two cholestatic mouse models did not display increased scratch activity as a measure of itch perception. Increasing plasma ATX activity by overexpression also did not lead to increased scratch activity in mice. This questions whether mice are suitable to study cholestatic itch.

Cholestasis-Associated Pruritus and Its Pruritogens

Pruritus is a debilitating symptom of various cholestatic disorders, including primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC) and inherited progressive familial intrahepatic cholestasis (PFIC). The molecular mechanisms leading to cholestasis-associated pruritus are still unresolved and the involved pruritogens are indecisive. As a consequence of pruritus, patients suffer from sleep deprivation, loss of daytime concentration, auto-mutilation and sometimes even suicidal ideations. Current guideline-approved therapy of cholestasis-associated pruritus includes stepwise administration of several medications, which may alleviate complaints in some, but not all affected patients. Therefore, also experimental therapeutic approaches are required to improve patients' quality of life. This article reviews the current state of research on pruritogens and their receptors, and shortly discusses the most recent experimental therapies.

Unaltered Liver Regeneration in Post-Cholestatic Rats Treated with the FXR Agonist Obeticholic Acid

In a previous study, obeticholic acid (OCA) increased liver growth before partial hepatectomy (PHx) in rats through the bile acid receptor farnesoid X-receptor (FXR). In that model, OCA was administered during obstructive cholestasis. However, patients normally undergo PHx several days after biliary drainage. The effects of OCA on liver regeneration were therefore studied in post-cholestatic Wistar rats. Rats underwent sham surgery or reversible bile duct ligation (rBDL), which was relieved after 7 days. PHx was performed one day after restoration of bile flow. Rats received 10 mg/kg OCA per day or were fed vehicle from restoration of bile flow until sacrifice 5 days after PHx. Liver regeneration was comparable between cholestatic and non-cholestatic livers in PHx-subjected rats, which paralleled liver regeneration a human validation cohort. OCA treatment induced ileal Fgf15 mRNA expression but did not enhance post-PHx hepatocyte proliferation through FXR/SHP signaling. OCA treatment neither increased mitosis rates nor recovery of liver weight after PHx but accelerated liver regrowth in rats that had not been subjected to rBDL. OCA did not increase biliary injury. Conclusively, OCA does not induce liver regeneration in post-cholestatic rats and does not exacerbate biliary damage that results from cholestasis. This study challenges the previously reported beneficial effects of OCA in liver regeneration in cholestatic rats.

A Novel Fluorescence-Based Method to Evaluate Ileal Apical Sodium-Dependent Bile Acid Transporter ASBT

This study aimed to demonstrate usefulness of the fluorophore-labeled bile acid derivative, N-(24-[7-(4-N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole)]amino-3α,7α,12α-trihydroxy-27-nor-5β-cholestan-26-oyl)-2'-aminoethane sulfonate (tauro-nor-THCA-24-DBD) as a substrate of apical sodium-dependent bile acid transporter (ASBT, SLC10A2), which is expressed at distal ileum for reabsorption of bile acids and to find a novel fluorescence-based method to evaluate ASBT activity. In HPLC analysis, chromatogram of tauro-nor-THCA-24-DBD showed double peaks: R- and S-isomers of the compound. When ASBT was expressed in Xenopus laevis oocytes, their uptakes were higher than those by control oocytes, demonstrating both are transported by ASBT. Therefore, results were analyzed separately as peak 1, peak 2 and sum of them. Concentration dependent uptake of tauro-nor-THCA-24-DBD in ASBT-expressing oocytes was saturable with Km 122 μM and Vmax 1.49 pmol/oocyte/30 min for peak 1, 30.7 μM and 1.34 pmol/oocyte/30 min for peak 2, and 40.6 μM and 2.36 pmol/oocyte/30 min for sum, respectively. These uptakes were decreased in the presence of taurocholic acid and in the Na⁺ free condition. Furthermore, in Caco-2 cells, tauro-nor-THCA-24-DBD uptake was also Na⁺-dependent and saturable. Additionally, these uptakes were decreased by elobixibat, a selective ASBT inhibitor. Accordingly, it was concluded that tauro-nor-THCA-24-DBD is a substrate of ASBT and useful to evaluate the intestinal ASBT transport activity.

Pregnancy complicated by refractory severe hypercholanaemia from sodium taurocholate co-transporting polypeptide deficiency

Sodium taurocholate co-transporting polypeptide deficiency is a rare metabolic autosomal recessive condition resulting in critically elevated plasma bile acid levels. Hypercholanaemia in similar conditions such as intrahepatic cholestasis of pregnancy has been associated with an increased risk of adverse obstetric outcomes including stillbirth. We present the first case of Sodium taurocholate co-transporting polypeptide deficiency in a current pregnancy in a patient with one previous stillbirth in the context of severe hypercholanaemia, where conventional treatments for cholestasis including ursodeoxycholic acid, rifampicin and cholestyramine were ineffective. Therapeutic plasma exchange and novel treatment with elobixibat were trialed with mixed results. The pregnancy resulted in an iatrogenic preterm delivery of a live infant at 32 weeks gestation.

Abnormal Bilirubin Metabolism in Patients With Sodium Taurocholate Cotransporting Polypeptide Deficiency

OBJECTIVES

The aim of the study was to explore the significance of sodium taurocholate cotransporting polypeptide (NTCP) deficiency and its clinical features in Chinese children presenting with isolated persistent hypercholanemia.

METHODS

The exon and adjacent regions of SLC10A1, the gene encoding NTCP, were sequenced in 33 Chinese children presenting with isolated hypercholanemia. Clinical history and medical data were reviewed. Growth milestones were compared with the national standard. The serum direct bilirubin concentration at last follow-up was compared with age- and sex-matched controls.

RESULTS

A variant, c.800C>T, p. S267F of SLC10A1 was detected in all subjects; 30 patients were homozygotes and 3 were compound heterozygotes. Nine patients presented with transient neonatal cholestasis, and 1 with a persistent mild conjugated hyperbilirubinemia. The serum direct bilirubin level in NTCP-deficient patients was significantly higher than age- and sex-matched controls even after the neonatal cholestasis stage (2.85 ± 1.50 vs 1.49 ± 0.70 μmol/L, P = 0.00008). No growth delay or other severe long-term clinical consequences were observed.

CONCLUSIONS

NTCP deficiency is the exclusive or major cause of isolated hypercholanemia in Han Chinese children, with c.800C>T the major contributing genetic variation. The defect may affect bilirubin metabolism and present as transient neonatal cholestasis and/or persistent mild conjugated hyperbilirubinmia, but with no apparent long-term clinical consequences.

Metabolic consequences of ileal interruption of the enterohepatic circulation of bile acids

The enterohepatic circulation of bile acids comprises a tightly regulated process of hepatic bile acid secretion, intestinal reabsorption and transport back to the liver. Disruption of this process has significant consequences for gastrointestinal, liver and whole body homeostasis and therefore offers opportunities for therapeutic intervention. In this review we discuss the effects of (pharmacological) interruption of the enterohepatic circulation at different levels. Recently, several studies have been published on ileal interruption of the enterohepatic circulation of bile acids, targeting the apical-sodium dependent bile acid transporter (ASBT, SLC10A2), as therapy for various diseases. However, ambiguous results have been reported and in-depth mechanistic insights are lacking. Here we discuss these novel studies and review the current knowledge on the consequences of ASBT inhibition and its potential effects on physiology and metabolism.

Spontaneous liver disease in wild-type C57BL/6JOlaHsd mice fed semisynthetic diet

Mouse models are frequently used to study mechanisms of human diseases. Recently, we observed a spontaneous bimodal variation in liver weight in C57BL/6JOlaHsd mice fed a semisynthetic diet. We now characterized the spontaneous variation in liver weight and its relationship with parameters of hepatic lipid and bile acid (BA) metabolism. In male C57BL/6JOlaHsd mice fed AIN-93G from birth to postnatal day (PN)70, we measured plasma BA, lipids, Very low-density lipoprotein (VLDL)-triglyceride (TG) secretion, and hepatic mRNA expression patterns. Mice were sacrificed at PN21, PN42, PN63 and PN70. Liver weight distribution was bimodal at PN70. Mice could be subdivided into two nonoverlapping groups based on liver weight: 0.6 SD 0.1 g (approximately one-third of mice, small liver; SL), and 1.0 SD 0.1 g (normal liver; NL; p<0.05). Liver histology showed a higher steatosis grade, inflammation score, more mitotic figures and more fibrosis in the SL versus the NL group. Plasma BA concentration was 14-fold higher in SL (p<0.001). VLDL-TG secretion rate was lower in SL mice, both absolutely (-66%, p<0.001) and upon correction for liver weight (-44%, p<0.001). Mice that would later have the SL-phenotype showed lower food efficiency ratios during PN21-28, suggesting the cause of the SL phenotype is present at weaning (PN21). Our data show that approximately one-third of C57BL/6JOlaHsd mice fed semisynthetic diet develop spontaneous liver disease with aberrant histology and parameters of hepatic lipid, bile acid and lipoprotein metabolism. Study designs involving this mouse strain on semisynthetic diets need to take the SL phenotype into account. Plasma lipids may serve as markers for the identification of the SL phenotype.

Recent advances in understanding the molecular mechanisms of cholestatic pruritus: A review

Cholestasis, a condition characterized by an abnormal decrease in bile flow, is accompanied by various symptoms such as pruritus. Although cholestatic pruritus is a prominent condition, its precise mechanisms have largely been elusive. Recently, advancements have been made for understanding the etiology and pathogenesis of cholestatic pruritus. The current review therefore focuses on summarizing the overall progress made in the elucidation of its molecular mechanisms. We have reviewed the available animal models on cholestasis to compare the differences between them, characterized potential pruritogens involved in cholestatic pruritus, and have summarized the receptor and ion channels implicated in the condition. Finally, we have discussed the available treatment options for alleviation of cholestatic pruritus. As our understanding of the mechanisms of cholestatic pruritus deepens, novel strategies to cure this condition are awaited.

Use of a Bile Salt Export Pump Knockdown Rat Susceptibility Model to Interrogate Mechanism of Drug-Induced Liver Toxicity

Inhibition of the bile salt export pump (BSEP) may be associated with clinical drug-induced liver injury, but is poorly predicted by preclinical animal models. Here we present the development of a novel rat model using siRNA knockdown (KD) of Bsep that displayed differentially enhanced hepatotoxicity to 8 Bsep inhibitors and not to 3 Bsep noninhibitors when administered at maximally tolerated doses for 7 days. Bsep KD alone resulted in 3- and 4.5-fold increases in liver and plasma levels, respectively, of the sum of the 3 most prevalent taurine conjugated bile acids (T3-BA), approximately 90% decrease in plasma and liver glycocholic acid, and a distinct bile acid regulating gene expression pattern, without resulting in hepatotoxicity. Among the Bsep inhibitors, only asunaprevir and TAK-875 resulted in serum transaminase and total bilirubin increases associated with increases in plasma T3-BA that were enhanced by Bsep KD. Benzbromarone, lopinavir, and simeprevir caused smaller increases in plasma T3-BA, but did not result in hepatotoxicity in Bsep KD rats. Bosentan, cyclosporine A, and ritonavir, however, showed no enhancement of T3-BA in plasma in Bsep KD rats, as well as Bsep noninhibitors acetaminophen, MK-0974, or clarithromycin. T3-BA findings were further strengthened through monitoring TCA-d4 converted from cholic acid-d4 overcoming interanimal variability in endogenous bile acids. Bsep KD also altered liver and/or plasma levels of asunaprevir, TAK-875, TAK-875 acyl-glucuronide, benzbromarone, and bosentan. The Bsep KD rat model has revealed differences in the effects on bile acid homeostasis among Bsep inhibitors that can best be monitored using measures of T3-BA and TCA-d4 in plasma. However, the phenotype caused by Bsep inhibition is complex due to the involvement of several compensatory mechanisms.

Very High Dehydroepiandrosterone Sulfate (DHEAS) in Serum of an Overweight Female Adolescent Without a Tumor

Introduction: An increase of serum dehydroepiandrosterone (DHEA) sulfate (DHEAS) is observed in premature adrenarche and congenital adrenal hyperplasia. Very high DHEAS levels are typical for adrenal tumors. Approximately 74% of DHEAS is hydrolyzed to DHEA by the steroid sulfatase (STS). The reverse reaction is DHEA sulfation. Besides these two enzyme reactions, the DHEAS transported through the cell membrane is important for its distribution and excretion. Case Presentation: We present a female adolescent with overweight and a very high DHEAS. The presence of a DHEAS-producing tumor was rejected using ultrasonography, Magnetic Resonance Tomography (MRT), and dexamethasone suppression. STS deficiency was suspected. Sequence analysis revealed a heterozygous nonsense mutation which predicts a truncation of the carboxyl region of the STS that is implicated in substrate binding. No partial gene deletion outside exon 5 was detected by multiplex ligation-dependent probe amplification. The bioassay revealed normal enzyme activity in the patient's leukocytes. A defect of transporter proteins was suggested. Both efflux [multidrug-resistance protein (MRP)2 and breast cancer-resistance protein (BCRP)] and uptake [organic anion-transporting polypeptide (OATP) and organic anion transporter (OAT) carriers] transporters were studied. Sequence analysis of exons revealed a heterozygous Q141K variant for BCRP. Conclusions: A novel heterozygous nonsense mutation in the STS gene and a known heterozygous missense variant in the BCRP gene were found. The heterozygous nonsense mutation in the STS gene is not supposed to be responsible for STS deficiency. The BCRP variant is associated with reduced efflux transport activity only in its homozygous state. The combination of the two heterozygous mutations could possibly explain the observed high levels of DHEAS and other sulfated steroids.

The Lipid Raft Component Stomatin Interacts with the Na+ Taurocholate Cotransporting Polypeptide (NTCP) and Modulates Bile Salt Uptake

The sodium taurocholate cotransporting polypeptide (NTCP) is expressed at the basolateral membrane of hepatocytes, where it mediates the uptake of conjugated bile acids and forms the hepatocyte entry receptor for the hepatitis B and D virus. Here, we aimed to identify novel protein–protein interactions that could play a role in the regulation of NTCP. To this end, NTCP was precipitated from HA-tagged hNTCP-expressing HepG2 cells, and chloride channel CLIC-like 1 (CLCC1) and stomatin were identified as interacting proteins by mass spectrometry. Interaction was confirmed by co-immunoprecipitation. NTCP, CLCC1 and stomatin were found at the plasma membrane in lipid rafts, as demonstrated by a combination of immunofluorescence, cell surface biotinylation and isolation of detergent-resistant membranes. Neither CLCC1 overexpression nor its knockdown had an effect on NTCP function. However, both stomatin overexpression and knockdown increased NTCP-mediated taurocholate uptake while NTCP abundance at the plasma membrane was only increased in stomatin depleted cells. These findings identify stomatin as an interactor of NTCP and show that the interaction modulates bile salt transport.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Inflammation: Cause or consequence of chronic cholestatic liver injury

Cholestasis is a result of obstruction of the biliary tracts. It is a common cause of liver pathology after exposure to toxic xenobiotics and during numerous other liver diseases. Accumulation of bile acids in the liver is thought to be a major driver of liver injury during cholestasis and can lead to eventual liver fibrosis and cirrhosis. As such, current therapy in the field of chronic liver diseases with prominent cholestasis relies heavily on increasing choleresis to limit accumulation of bile acids. Many of these same diseases also present with autoimmunity before the onset of cholestasis though, indicating the inflammation may be an initiating component of the pathology. Moreover, cytotoxic inflammatory mediators accumulate during cholestasis and can propagate liver injury. Anti-inflammatory biologics and small molecules have largely failed clinical trials in these diseases though and as such, targeting inflammation as a means to address cholestatic liver injury remains debatable. The purpose of this review is to understand the different roles that inflammation can play during cholestatic liver injury and attempt to define how new therapeutic targets that limit or control inflammation may be beneficial for patients with chronic cholestatic liver disease.

Inhibition of Hepatic Bile Acid Uptake by Myrcludex B Promotes Glucagon-Like Peptide-1 Release and Reduces Obesity

BACKGROUND & AIMS

Bile acids are important metabolic signaling molecules. Bile acid receptor activation promotes body weight loss and improves glycemic control. The incretin hormone GLP-1 and thyroid hormone activation of T4 to T3 have been suggested as important contributors. Here, we identify the hepatic bile acid uptake transporter Na⁺ taurocholate co-transporting polypeptide (NTCP) as target to prolong postprandial bile acid signaling.

METHODS

Organic anion transporting polypeptide (OATP)1a/1b KO mice with or without reconstitution with human OATP1B1 in the liver were treated with the NTCP inhibitor Myrcludex B for 3.5 weeks after the onset of obesity induced by high fat diet-feeding. Furthermore, radiolabeled T4 was injected to determine the role of NTCP and OATPs in thyroid hormone clearance from plasma.

RESULTS

Inhibition of NTCP by Myrcludex B in obese Oatp1a/1b KO mice inhibited hepatic clearance of bile acids from portal and systemic blood, stimulated GLP-1 secretion, reduced body weight, and decreased (hepatic) adiposity. NTCP inhibition did not affect hepatic T4 uptake nor lead to increased thyroid hormone activation. Myrcludex B treatment increased fecal energy output, explaining body weight reductions amongst unaltered food intake and energy expenditure.

CONCLUSIONS

Pharmacologically targeting hepatic bile acid uptake to increase bile acid signaling is a novel approach to treat obesity and induce GLP1- secretion.

The Role of the Sodium-taurocholate Co-transporting Polypeptide (NTCP) and Bile Salt Export Pump (BSEP) in Related Liver Disease

BACKGROUND

Sodium Taurocholate Co-transporting Polypeptide (NTCP) and Bile Salt Export Pump (BSEP) play significant roles as membrane transporters because of their presence in the enterohepatic circulation of bile salts. They have emerged as promising drug targets in related liver disease.

METHODS

We reviewed the literature published over the last 20 years with a focus on NTCP and BSEP.

RESULTS

This review summarizes the current perception about structure, function, genetic variation, and regulation of NTCP and BSEP, highlights the effects of their defects in some hepatic disorders, and discusses the application prospect of new transcriptional activators in liver diseases.

CONCLUSION

NTCP and BSEP are important proteins for transportation and homeostasis maintenance of bile acids. Further research is needed to develop new models for determining the structure-function relationship of bile acid transporters and screening for substrates and inhibitors, as well as to gain more information about the regulatory genetic mechanisms involved in the processes of liver injury.

Present and Future Therapies for Chronic Hepatitis B

Chronic hepatitis B (CHB) remains the leading cause of liver-related morbidity and mortality across the world. If left untreated, approximately one-third of these patients will progress to severe end-stage liver diseases including liver failure, cirrhosis, and hepatocellular carcinoma (HCC). High level of serum HBV DNA is strongly associated with the development of liver failure, cirrhosis, and HCC. Therefore, antiviral therapy is crucial for the clinical management of CHB. Current antiviral drugs including nucleoside/nucleotide analogues (NAs) and interferon-α (IFN-α) can suppress HBV replication and reduce the progression of liver disease, thus improving the long-term outcomes of CHB patients. This chapter will discuss the standard and optimization antiviral therapies in treatment-naïve and treatment-experienced patients, as well as in the special populations. The up-to-date advances in the development of new anti-HBV agents will be also discussed. With the combination of the current antiviral drugs and the newly developed antiviral agents targeting the different steps of the viral life cycle or the newly developed agents modulating the host immune responses, the ultimate eradication of HBV will be achieved in the future.

Sodium Taurocholate Cotransporting Polypeptide (NTCP) Deficiency Hidden Behind Citrin Deficiency in Early Infancy: A Report of Three Cases

Sodium taurocholate cotransporting polypeptide (NTCP), a carrier protein encoded by the gene SLC10A1, is expressed in the basolateral membrane of the hepatocyte to uptake bile acids from plasma. As a new inborn error of bile acid metabolism, NTCP deficiency remains far from being well understood in terms of the clinical and molecular features. Citrin deficiency is a well-known autosomal recessive disease arising from SLC25A13 mutations, and in neonates or infants, this condition presents as transient intrahepatic cholestasis which usually resolves before 1 year of age. All the three patients in this paper exhibited cholestatic jaundice and elevated total bile acids in their early infancy, which were attributed to citrin deficiency by SLC25A13 genetic analysis. In response to feeding with lactose-free and medium-chain triglycerides-enrich formula, their clinical and laboratory presentations disappeared gradually while the hypercholanemia persisted, even beyond 1 year of age. On subsequent SLC10A1 analysis, they were all homozygous for the well-known pathogenic variant c.800C > T (p.Ser267Phe), and NTCP deficiency was thus definitely diagnosed. The findings in this paper indicated that NTCP deficiency could be covered up by citrin deficiency during early infancy; however, in citrin-deficient patients with intractable hypercholanemia following resolved cholestatic jaundice, NTCP deficiency should be taken into consideration.

Inflammation and Cell Death During Cholestasis: The Evolving Role of Bile Acids

Cholestasis results in blockage of bile flow whether the point of obstruction occurs extrahepatically or intrahepatically. Bile acids are a primary constituent of bile, and thus one of the primary outcomes is acute retention of bile acids in hepatocytes. Bile acids are normally secreted into the biliary tracts and then released into the small bowel before recirculating back to the liver. Retention of bile acids has long been hypothesized to be a primary cause of the associated liver injury that occurs during acute or chronic cholestasis. Despite this, a surge of papers in the last decade have reported a primary role for inflammation in the pathophysiology of cholestatic liver injury. Furthermore, it has increasingly been recognized that both the constituency of individual bile acids that make up the greater pool, as well as their conjugation status, is intimately involved in their toxicity, and this varies between species. Finally, the role of bile acids in drug-induced cholestatic liver injury remains an area of increasing interest. The purpose of this review is to critically evaluate current proposed mechanisms of cholestatic liver injury, with a focus on the evolving role of bile acids in cell death and inflammation.

Clinical and molecular characterization of four patients with NTCP deficiency from two unrelated families harboring the novel SLC10A1 variant c.595A>C (p.Ser199Arg)

Sodium taurocholate cotransporting polypeptide (NTCP), a carrier protein encoded by solute carrier family 10 member 1 (SLC10A1), is expressed in the basolateral membrane of hepatocytes, where it is responsible for the uptake of bile acids from plasma into hepatocytes. The first patient with NTCP deficiency was described in 2015. A limited number of such patients have been reported in the literature and their genotypic and phenotypic features require further investigation. The current study investigated 4 patients with NTCP deficiency from two unrelated families. The patients were subjected to SLC10A1 genetic analysis and it was revealed that all patients were compound heterozygous for the c.800C>T (p.Ser267Phe) and c.595A>C (p.Ser199Arg) SLC10A1 variants. To the best of the authors' knowledge, the latter variant had not been previously reported. Further analysis in 50 healthy individuals did not identify carriers. The c.595A>C (p.Ser199Arg) variant exhibited co-segregation with hypercholanemia and exhibited a relatively conserved amino acid when compared with homologous peptides. Moreover, SWISS-MODEL prediction revealed that the mutation affected the conformation of the NTCP molecule. The 4 patients demonstrated varying degrees of hypercholanemia while a downward trend in the plasma levels of total bile acids (TBA) in 2 pediatric patients and occasionally normal TBA level in an adult case were observed. The results indicated an autosomal recessive trait for NTCP deficiency, supported the primary role of NTCP in the uptake of bile acids from plasma and suggested that hepatic uptake of bile acids may occur by means other than NTCP uptake. Moreover, the novel missense variant c.595A>C(p.Ser199Arg) enriched the SLC10A1 mutation spectrum and may serve as a new genetic marker for the molecular diagnosis and genetic counseling of NTCP deficiency.

Can Bile Salt Export Pump Inhibition Testing in Drug Discovery and Development Reduce Liver Injury Risk? An International Transporter Consortium Perspective

Bile salt export pump (BSEP) inhibition has emerged as an important mechanism that may contribute to the initiation of human drug‐induced liver injury (DILI). Proactive evaluation and understanding of BSEP inhibition is recommended in drug discovery and development to aid internal decision making on DILI risk. BSEP inhibition can be quantified using in vitro assays. When interpreting assay data, it is important to consider in vivo drug exposure. Currently, this can be undertaken most effectively by consideration of total plasma steady state drug concentrations (C_ss,plasma). However, because total drug concentrations are not predictive of pharmacological effect, the relationship between total exposure and BSEP inhibition is not causal. Various follow‐up studies can aid interpretation of in vitro BSEP inhibition data and may be undertaken on a case‐by‐case basis. BSEP inhibition is one of several mechanisms by which drugs may cause DILI, therefore, it should be considered alongside other mechanisms when evaluating possible DILI risk.

Clinical and histopathologic features of sodium taurocholate cotransporting polypeptide deficiency in pediatric patients

Until now, the recognition of sodium taurocholate cotransporting polypeptide (NTCP) deficiency has been mainly based on sporadic case reports. It was previously believed to be mildly symptomatic and resulting in mild liver dysfunction. However, to our knowledge, there have been no reports about the histopathologic and ultrastructural pathologic characteristics of the disease. The aim of the study was to analyze the clinical, histopathologic and ultrastructural pathologic characteristics of NTCP deficiency in 13 pediatric patients.From August 2012 to October 2018, this retrospective study conducted in the Department of Pediatrics of Tongji Hospital, China analyzed the data of 13 NTCP deficient patients with an SLC10A1 gene mutation. Except for NTCP deficiency, no other liver diseases were present in the patients, which was determined by both a genetic testing panel for jaundice and by reviewing medical records. The laboratory results, imaging, histopathologic, and ultrastructural pathologic information were recorded for analysis.The serum level of total bile acid was high in all 13 patients. All patients had adequate growth and development. Eight of the patients (8/13) presented with visible jaundice and 12 (12/13) were found to have hyperbilirubinemia. A needle liver biopsy was performed in 11 cases, which revealed slightly chronic inflammation in all 11 patients. One of the patients (1/13) was found to be suffering from gallstones.The data showed that although NTCP deficiency was often asymptomatic, some of the patients showed obvious clinical expressions, such as jaundice. Among the 13 pediatric patients with NTCP deficiency, both the biochemical and histopathologic features were similar to those of mild hepatocellular jaundice. In addition, it was determined that the clinical features in the patient with gallstones may have been caused by NTCP deficiency.

NTCP deficiency in mice protects against obesity and hepatosteatosis

Bile acids play a major role in the regulation of lipid and energy metabolism. Here we propose the hepatic bile acid uptake transporter Na⁺ taurocholate cotransporting polypeptide (NTCP) as a target to prolong postprandial bile acid elevations in plasma. Reducing hepatic clearance of bile acids from plasma by genetic deletion of NTCP moderately increased plasma bile acid levels, reduced diet-induced obesity, attenuated hepatic steatosis, and lowered plasma cholesterol levels. NTCP and G protein–coupled bile acid receptor–double KO (TGR5–double KO) mice were equally protected against diet-induced obesity as NTCP–single KO mice. NTCP-KO mice displayed decreased intestinal fat absorption and a trend toward higher fecal energy output. Furthermore, NTCP deficiency was associated with an increased uncoupled respiration in brown adipose tissue, leading to increased energy expenditure. We conclude that targeting NTCP-mediated bile acid uptake can be a novel approach to treat obesity and obesity-related hepatosteatosis by simultaneously dampening intestinal fat absorption and increasing energy expenditure.

Targeting bile acid uptake simultaneously dampens intestinal fat absorption and increases energy expenditure, suggesting a potential approach to treat obesity and obesity-related hepatosteatosis.

Increased sulfation of bile acids in mice and human subjects with sodium taurocholate cotransporting polypeptide deficiency

Sodium taurocholate cotransporting polypeptide (NTCP, encoded by Slc10a1/SLC10A1) deficiency can result in hypercholanemia but no obvious symptoms in both mice and humans. However, the consequence of and response to long-term hypercholanemia caused by NTCP deficiency remain largely unexplored. Here, we analyzed lifelong dynamics of serum total bile acid (TBA) levels in Slc10a1 ^-/- mice, and we also assessed changes of TBA levels in 33 young individuals with SLC10A1 loss-of-function variant p.Ser267Phe. We found that overall serum TBA levels tended to decrease gradually with age in both Slc10a1 ^-/- mice and p.Ser267Phe individuals. Liver mRNA profiling revealed notable transcription alterations in hypercholanemic Slc10a1 ^-/- mice, including inhibition of bile acid (BA) synthesis, enhancement of BA detoxification, and altered BA transport. Members of the sulfotransferase (SULT) family showed the most dramatic increases in livers of hypercholanemic Slc10a1 ^-/- mice, and one of their BA sulfates, taurolithocholic acid 3-sulfate, significantly increased. Importantly, consistent with the mouse studies, comprehensive profiling of 58 BA species in sera of p.Ser267Phe individuals revealed a markedly increased level of BA sulfates. Together, our findings indicate that the enhanced BA sulfation is a major mechanism for BA detoxification and elimination in both mice and humans with Slc10a1/SLC10A1 deficiency.

Association of NTCP polymorphisms with clinical outcome of hepatitis B infection in Thai individuals

BACKGROUND

Single nucleotide polymorphisms (SNPs) in the sodium taurocholate co-transporting polypeptide (NTCP) have been showed to be associated with natural history of hepatitis B virus (HBV) infection. However, it is unclear whether the SNPs are related to the clinical outcome of HBV infection in Thai individuals.

METHODS

The rs2296651 and rs4646287 polymorphisms of NTCP were determined by allelic discrimination using commercial TaqMan probes in blood samples of 1021 Thai individuals. These subjects included 610 patients with chronic HBV infection [CHB, 305 with hepatocellular carcinoma (HCC) and 305 without HCC], 206 subjects with spontaneous HBV clearance and 205 healthy controls who were age and gender-matched.

RESULTS

The frequencies of rs2296651 A minor allele in the CHB group, the HBV clearance group and healthy controls were 7.8, 7.3 and 13.9%, respectively. For rs4646287, the frequencies of T minor allele of the corresponding groups were 10.4, 8.0 and 9.5%, respectively. Compared with healthy controls, the frequencies of rs2296651 GA + AA genotypes were significantly lower in the CHB group (P &lt; 0.001) and in the HBV clearance group (P = 0.001). There was no difference in their distribution between the HBV clearance and CHB groups. Among the CHB group, the distribution of GA + AA genotypes in patients with HCC were significantly lower than in patients without HCC (P = 0.014). The frequencies of HBeAg positivity in patients harboring GG and GA + AA genotypes were 39.8 and 23.5%, respectively (P = 0.004). Among patients with HCC, the mean HBV DNA of the corresponding genotypes were 4.9 ± 1.3 vs. 2.7 ± 1.0 log₁₀ IU/mL, respectively (P &lt; 0.001). There was no difference in genotype and allele frequencies of rs4646287 polymorphism among all studied groups.

CONCLUSIONS

Our results showed that rs2296651 polymorphism was associated with a decreased risk of susceptibility to HBV infection and the development of HCC. These data suggest that the NTCP polymorphism might have an influence on natural history of HBV infection in Thai individuals. This abstract was partly presented at the American Association for the Study of Liver Diseases (AASLD) Meeting 2018, November 9-13, 2018, in San Francisco, CA, USA and was published in Hepatology 2018; 68:1237A-1238A.

NTCP S267F variant associates with decreased susceptibility to HBV and HDV infection and decelerated progression of related liver diseases

OBJECTIVES

To determine potential associations of the rs2296651 variant (c.800C>T, S267F) of NTCP with HBV and HBV plus concomitant HDV infection as well as with the progression of related liver diseases.

METHODS

The S267F variant was genotyped by DNA sequencing in 620 HBV-infected patients and 214 healthy controls (HCs). Among the patients, 450 individuals were tested for HDV by a nested PCR assay. Logistic regression was applied to examine the association.

RESULTS

The S267F variant was found more frequently among HCs (16%) compared to HBV-infected (6%) and HBV-HDV co-infected patients (3%) (HBV patients vs HC: OR=0.32, P=0.00002 and HDV patients vs. HC: OR=0.17, P=0.018). The frequency of S267F variant was inversely correlated with CHB, LC or HCC patients compared with HCs (OR=0.31, P=0.001; OR=0.32, P=0.013; OR=0.34, P=0.002, respectively). S267F variant was also associated with decreased risk of the development of advanced liver cirrhosis (LC) and hepatocellular carcinoma (HCC) (Child B and C vs. Child A, OR=0.26, adjusted P=0.016; BCLC B,C,D vs. BCLC A, OR=0.038, P=0.045, respectively). In addition, patients with the genotype CT had lower levels of AST, ALT, total and direct bilirubin as well as higher platelet counts, indicating an association with a more favorable clinical outcome.

CONCLUSION

The NTCP S267F variant of the SLC10A1 gene exhibits protective effects against HBV and HDV infection and is associated with a reduced risk of developing to advanced stages of LC and HCC.

Guts and Gall: Bile Acids in Regulation of Intestinal Epithelial Function in Health and Disease

Epithelial cells line the entire surface of the gastrointestinal tract and its accessory organs where they primarily function in transporting digestive enzymes, nutrients, electrolytes, and fluid to and from the luminal contents. At the same time, epithelial cells are responsible for forming a physical and biochemical barrier that prevents the entry into the body of harmful agents, such as bacteria and their toxins. Dysregulation of epithelial transport and barrier function is associated with the pathogenesis of a number of conditions throughout the intestine, such as inflammatory bowel disease, chronic diarrhea, pancreatitis, reflux esophagitis, and cancer. Driven by discovery of specific receptors on intestinal epithelial cells, new insights into mechanisms that control their synthesis and enterohepatic circulation, and a growing appreciation of their roles as bioactive bacterial metabolites, bile acids are currently receiving a great deal of interest as critical regulators of epithelial function in health and disease. This review aims to summarize recent advances in this field and to highlight how bile acids are now emerging as exciting new targets for disease intervention.

Developments in bile salt based therapies: A critical overview

Bile acids, amphipathic molecules known for their facilitating role in fat absorption, are also recognized as signalling molecules acting via nuclear and membrane receptors. Of the bile acid-activated receptors, the Farnesoid X Receptor (FXR) and the G protein-coupled bile acid receptor-1 (Gpbar1 or TGR5) have been studied most extensively. Bile acid signaling is critical in the regulation of bile acid metabolism itself, but it also plays a significant role in glucose, lipid and energy metabolism. Activation of FXR and TGR5 leads to reduced hepatic bile salt load, improved insulin sensitivity and glucose regulation, increased energy expenditure, and anti-inflammatory effects. These beneficial effects render bile acid signaling an interesting therapeutic target for the treatment of diseases such as cholestasis, non-alcoholic fatty liver disease, and diabetes. Here, we summarize recent findings on bile acid signaling and discuss potential and current limitations of bile acid receptor agonist and modulators of bile acid transport as future therapeutics for a wide-spectrum of diseases.

The functional role of sodium taurocholate cotransporting polypeptide NTCP in the life cycle of hepatitis B, C and D viruses

Chronic hepatitis B, C and D virus (HBV, HCV and HDV) infections are a major cause of liver disease and cancer worldwide. Despite employing distinct replication strategies, the three viruses are exclusively hepatotropic, and therefore depend on hepatocyte-specific host factors. The sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes that mediates the transport of bile acids, plays a key role in HBV and HDV entry into hepatocytes. Recently, NTCP has been shown to modulate HCV infection of hepatocytes by regulating innate antiviral immune responses in the liver. Here, we review the current knowledge of the functional role and the molecular and cellular biology of NTCP in the life cycle of the three major hepatotropic viruses, highlight the impact of NTCP as an antiviral target and discuss future avenues of research.

Calnexin Depletion by Endoplasmic Reticulum Stress During Cholestasis Inhibits the Na+-Taurocholate Cotransporting Polypeptide

Cholestasis‐induced accumulation of bile acids in the liver leads to farnesoid X receptor (FXR)‐mediated transcriptional down‐regulation of the bile acid importer Na⁺‐taurocholate cotransporting protein (NTCP) and to induction of endoplasmic reticulum (ER) stress. However, whether ER stress affects bile acid uptake is largely unknown. Here, we investigated the role of ER stress on the regulation and function of the bile acid transporter NTCP. ER stress was induced using thapsigargin or subtilase cytotoxin in human osteosarcoma (U2OS) and human hepatocellular carcinoma (HepG2) cells stably expressing NTCP. Cellular bile acid uptake was determined using radiolabeled taurocholate (TCA). NTCP plasma membrane expression was determined by cell surface biotinylation. Mice received a single injection of thapsigargin, and effects of ER stress on NTCP messenger RNA (mRNA) and protein were measured by reverse‐transcription polymerase chain reaction (RT‐PCR) and western blot analysis. Effects of cholestasis on NTCP and ER stress were assessed in response to 3, 5‐diethoxycarbonyl‐1, 4‐dihydrocollidine (DDC) feeding or bile duct ligation in FXR^–/– mice after 7 or 3 days, respectively. Novel NTCP‐interacting proteins were identified by mass spectrometry (MS), interaction verified, and assessed by co‐immunoprecipitation and TCA uptake for functional relevance in relation to ER stress. ER stress induction strongly reduced NTCP protein expression, plasma membrane abundance, and NTCP‐mediated bile acid uptake. This was not controlled by FXR or through a single unfolded protein response (UPR) pathway but mainly depended on the interaction of NTCP with calnexin, an ER chaperone. In mice, expression of both NTCP and calnexin was reduced by thapsigargin or cholestasis‐induced ER stress. Calnexin down‐regulation in vitro recapitulated the effect of ER stress on NTCP. Conclusion: ER stress‐induced down‐regulation of calnexin provides an additional mechanism to dampen NTCP‐mediated bile acid uptake and protect hepatocytes against bile acid overload during cholestasis.

Yinchenhao Decoction Alleviates Liver Fibrosis by Regulating Bile Acid Metabolism and TGF-β/Smad/ERK Signalling Pathway

Yinchenhao decoction (YCHD), comprising Yinchenhao (Artemisiae Scopariae Herba), Zhizi (Gardeniae Fructus) and Dahuang (Radix Rhei et Rhizoma), is widely used for treating various diseases. We aimed to investigate the bile acid metabolic mechanism of YCHD in dimethylnitrosamine (DMN)-induced liver fibrosis model. Rats received DMN (10 mg/kg, intraperitoneally) for four successive weeks for liver fibrosis induction and were treated with YCHD for the last 2 weeks. Histopathological analysis showed that YCHD prevented DMN-induced histopathological changes in liver tissues. Serum liver function in YCHD group improved. Ultraperformance liquid chromatography-mass spectrometry analysis showed that YCHD significantly restored both free and conjugated bile acid levels increased by DMN, to normal levels. RT-qPCR results showed that YCHD treatment upregulated the expression of genes related to bile acid synthesis, reabsorption, and excretion. Western blotting analysis showed that YCHD downregulated α-SMA, TGF-β1, p-Smad3, and p-ERK1/2 expression in chenodeoxycholic acid (CDCA)-activated hepatic stellate cells (HSCs). The viability of CDCA-activated HSCs significantly increased after treatment with YCHD and PD98059 (an ERK inhibitor) compared to YCHD treatment alone. Our findings suggest that YCHD alleviated DMN-induced liver fibrosis by regulating enzymes responsible for bile acid metabolism. Additionally, it inhibits CDCA-induced HSC proliferation and activation via TGF-β1/Smad/ERK signalling pathway.

Advances in drug-induced cholestasis: Clinical perspectives, potential mechanisms and in vitro systems

Despite growing research, drug-induced liver injury (DILI) remains a serious issue of increasing importance to the medical community that challenges health systems, pharmaceutical industries and drug regulatory agencies. Drug-induced cholestasis (DIC) represents a frequent manifestation of DILI in humans, which is characterised by an impaired canalicular bile flow resulting in a detrimental accumulation of bile constituents in blood and tissues. From a clinical point of view, cholestatic DILI generates a wide spectrum of presentations and can be a diagnostic challenge. The drug classes mostly associated with DIC are anti-infectious, anti-diabetic, anti-inflammatory, psychotropic and cardiovascular agents, steroids, and other miscellaneous drugs. The molecular mechanisms of DIC have been investigated since the 1980s but they remain debatable. It is recognised that altered expression and/or function of hepatobiliary membrane transporters underlies some forms of cholestasis, and this and other concomitant mechanisms are very likely in DIC. Deciphering these processes may pave the ways for diagnosis, prognosis and prevention, for which currently major gaps and caveats exist. In this review, we summarise recent advances in the field of DIC, including clinical aspects, the potential mechanisms postulated so far and the in vitro systems that can be useful to investigate and identify new cholestatic drugs.

Na+ -taurocholate cotransporting polypeptide inhibition has hepatoprotective effects in cholestasis in mice

Accumulation of bile salts (BSs) during cholestasis leads to hepatic and biliary injury, driving inflammatory and fibrotic processes. The Na⁺ -Taurocholate Cotransporting Polypeptide (NTCP) is the major hepatic uptake transporter of BSs, and can be specifically inhibited by myrcludex B. We hypothesized that inhibition of NTCP dampens cholestatic liver injury. Acute cholestasis was induced in mice by a 3.5-diethoxycarbonyl-1.4-dihydrocollidine (DDC) diet or by bile duct ligation (BDL). Chronic cholestasis was investigated in Atp8b1-G308V and Abcb4/Mdr2 deficient mice. Mice were injected daily with myrcludex B or vehicle. Myrcludex B reduced plasma alkaline phosphatase (ALP) levels in DDC-fed, Atp8b1-G308V and BDL mice by 39%, 27% and 48% respectively. Expression of genes involved in fibrosis, proliferation and inflammation was reduced by myrcludex B treatment in DDC-fed and Atp8b1-G308V mice. NTCP-inhibition increased plasma BS levels from 604±277 to 1746±719 μm in DDC-fed mice, 432±280 to 762±288 μm in Atp8b1-G308V mice and from 522±130 to 3625±378 μm in BDL mice. NTCP-inhibition strongly aggravated weight loss in BDL mice, but not in other cholestatic models studied. NTCP-inhibition reduced biliary BS output in DDC-fed and Atp8b1-G308V mice by ∼50% while phospholipid (PL) output was maintained, resulting in a higher PL/BS ratio. Conversely, liver injury in Abcb4 deficient mice, lacking biliary phospholipid output, was aggravated after myrcludex B treatment. Conclusion: NTCP-inhibition by myrcludex B has hepatoprotective effects, by reducing BS load in hepatocytes and increasing the biliary PL/BS ratio. High micromolar plasma BS levels after NTCP-inhibition were well tolerated. NTCP-inhibition may be beneficial in selected forms of cholestasis. (Hepatology 2018).

N-Linked Glycosylation Is Not Essential for Sodium Taurocholate Cotransporting Polypeptide To Mediate Hepatitis B Virus Infection In Vitro

Sodium taurocholate cotransporting polypeptide (NTCP) has been identified as a hepatitis B virus (HBV) receptor, and its overexpression in HepG2 cell lines leads to efficient secretion of hepatitis B e antigen (HBeAg) following challenge with a large dose of cell culture-derived HBV (cHBV) particles. However, NTCP-reconstituted HepG2 cells are inefficiently infected by patient serum-derived HBV (sHBV) and release very little hepatitis B surface antigen (HBsAg) following cHBV infection, unlike differentiated HepaRG cells, which are naturally susceptible to both cHBV and sHBV particles. Here, we investigated whether NTCP could explain the different behaviors of the two cell types. Endogenous NTCP protein from differentiated HepaRG cells was unglycosylated despite wild-type coding sequence. HepaRG cells stably transfected with an epitope-tagged NTCP expression construct displayed higher sHBV but not cHBV susceptibility than cells transfected with the null mutant. Tagged NTCP introduced to both HepG2 and HepaRG cells was glycosylated, with N5 and N11 being sites of N-linked glycosylation. Mutating N5, N11, or both did not alter cell surface availability of NTCP or its subcellular localization, with both the singly glycosylated and nonglycosylated forms still capable of mediating cHBV infection in HepG2 cells. In conclusion, nonglycosylated NTCP is expressed by differentiated HepaRG cells and capable of mediating cHBV infection in HepG2 cells, but it cannot explain differential susceptibility of HepaRG and HepG2/NTCP cells to cHBV versus sHBV infection and different HBsAg/HBeAg ratios following cHBV infection. The responsible host factor(s) remains to be identified.IMPORTANCE HBV can infect differentiated HepaRG cells and also HepG2 cells overexpressing NTCP, the currently accepted HBV receptor. However, HepG2/NTCP cells remain poorly susceptible to patient serum-derived HBV particles and release very little hepatitis B surface antigen following infection by cell culture-derived HBV. We found differentiated HepaRG cells expressed nonglycosylated NTCP despite a wild-type coding sequence. NTCP introduced to HepG2 cells was glycosylated at two N-linked glycosylation sites, but mutating either or both sites failed to prevent infection by cell culture-derived HBV or to confer susceptibility to serum-derived HBV. Overexpressing NTCP in HepRG cells did not increase infection by cell culture-derived HBV or distort the ratio between the two viral antigens. These findings suggest that host factors unique to HepaRG cells are required for efficient infection by serum-derived HBV, and factors other than NTCP contribute to balanced viral antigen production following infection by cell culture-derived HBV.

New Insights in Genetic Cholestasis: From Molecular Mechanisms to Clinical Implications

Cholestasis is characterised by impaired bile secretion and accumulation of bile salts in the organism. Hereditary cholestasis is a heterogeneous group of rare autosomal recessive liver disorders, which are characterised by intrahepatic cholestasis, pruritus, and jaundice and caused by defects in genes related to the secretion and transport of bile salts and lipids. Phenotypic manifestation is highly variable, ranging from progressive familial intrahepatic cholestasis (PFIC)-with onset in early infancy and progression to end-stage liver disease-to a milder intermittent mostly nonprogressive form known as benign recurrent intrahepatic cholestasis (BRIC). Cases have been reported of initially benign episodic cholestasis that subsequently transitions to a persistent progressive form of the disease. Therefore, BRIC and PFIC seem to represent two extremes of a continuous spectrum of phenotypes that comprise one disease. Thus far, five representatives of PFIC (named PFIC1-5) caused by pathogenic mutations present in both alleles of ATP8B1, ABCB11, ABCB4, TJP2, and NR1H4 have been described. In addition to familial intrahepatic cholestasis, partial defects in ATP8B1, ABCB11, and ABCB4 predispose patients to drug-induced cholestasis and intrahepatic cholestasis in pregnancy. This review summarises the current knowledge of the clinical manifestations, genetics, and molecular mechanisms of these diseases and briefly outlines the therapeutic options, both conservative and invasive, with an outlook for future personalised therapeutic strategies.

Bona fide receptor for hepatitis B and D viral infections: Mechanism, research models and molecular drug targets

Hepatitis B infections have become a serious public health issue globally, and the current first-line antiviral treatment for this disease is not a true cure. Recently, sodium taurocholate cotransporting polypeptide (NTCP), a liver-specific bile acid transporter, was identified as a bona fide receptor for hepatitis B virus (HBV) and its satellite virus, hepatitis delta virus (HDV). Identification of the HBV receptor has led to the development of robust cell cultures and provides a potential target for new treatments. This review summarizes the process by which NTCP was discovered and describes its clinical significance as the receptor for HBV and HDV entry.

The p.Ser267Phe variant of sodium taurocholate cotransporting polypeptide (NTCP) supports HBV infection with a low efficiency

Sodium taurocholate cotransporting polypeptide (NTCP) is a functional receptor for human hepatitis B virus (HBV) and its satellite virus Hepatitis D virus (HDV). Physiologically, NTCP is responsible for the majority of sodium-dependent bile acids uptake by hepatocytes. The p.Ser267Phe (S267F) variant of NTCP is a single nucleotide polymorphism (SNP) previously found to cause substantial loss of ability to support HBV and HDV infection and its taurocholic acid uptake function in vitro. Intriguingly, ten individuals were identified as S267F homozygotes in population studies of chronic hepatitis B (CHB) patients. In this study, we identified new HBV isolates from one homozygous S267F mutation carrier and confirmed new isolates also use wildtype-NTCP as a cellular receptor. Furthermore, we demonstrated S267F variant of NTCP, though inefficient, is still a functional receptor for HBV entry. This study advances our understanding of NTCP-mediated HBV infection.

Review article: therapeutic bile acids and the risks for hepatotoxicity

BACKGROUND

Bile acids play important roles in cholesterol metabolism and signal through farnesoid X receptor and G protein-coupled receptors. Given their importance in liver biology, bile acid therapy enables therapeutic applications beyond the treatment of cholestatic liver disease. However, predicting hepatotoxicity of bile acids in humans is obscured due to inconsistent extrapolations of animal data to humans.

AIM

To review the evidence that could explain discordant bile acids hepatotoxicity observed in humans and animals.

METHOD

Literature search was conducted in PubMed using keywords "bile acid," "transporter," "hepatotoxicity," "clinical study," "animal study," "species difference," "mechanism," "genetic disorder." Relevant articles were selected for review.

RESULTS

Clinically significant hepatotoxicity was reported in response to certain bile acids, namely chenodeoxycholic acid, which was given a boxed warning for potential hepatotoxicity. The chemical structure, specifically the number and orientation of hydroxyl groups, significantly affects their hydrophobicity, an important factor in bile acid toxicity. Experimental studies show that hydrophobic bile acids can lead to liver injury through various mechanisms, such as death receptor signalling, mitochondrial dysfunction and inflammation. Although animal studies play a central role in investigating bile acid safety, there are considerable differences in bile acid composition, metabolism and hepatobiliary disposition across species. This does not allow appropriate safety inference, especially for predicting hepatotoxicity in humans. Exploring evidences stemming from inborn errors, genetic models of disease and toxicology studies further improves an understanding of bile acid hepatotoxicity.

CONCLUSION

Species differences should be considered in the development of bile acid related therapeutics. Although the mechanism of bile acid hepatotoxicity is still not fully understood, continued mechanistic studies will deepen our understanding.

Animal models to study bile acid metabolism

The use of animal models, particularly genetically modified mice, continues to play a critical role in studying the relationship between bile acid metabolism and human liver disease. Over the past 20 years, these studies have been instrumental in elucidating the major pathways responsible for bile acid biosynthesis and enterohepatic cycling, and the molecular mechanisms regulating those pathways. This work also revealed bile acid differences between species, particularly in the composition, physicochemical properties, and signaling potential of the bile acid pool. These species differences may limit the ability to translate findings regarding bile acid-related disease processes from mice to humans. In this review, we focus primarily on mouse models and also briefly discuss dietary or surgical models commonly used to study the basic mechanisms underlying bile acid metabolism. Important phenotypic species differences in bile acid metabolism between mice and humans are highlighted.

De Novo Macrocyclic Peptide Inhibitors of Hepatitis B Virus Cellular Entry

Hepatitis B virus (HBV) constitutes a significant public health burden, and currently available treatment options are not generally curative, necessitating the development of new therapeutics. Here we have applied random non-standard peptide integrated discovery (RaPID) screening to identify small macrocyclic peptide inhibitors of HBV entry that target the cell-surface receptor for HBV, sodium taurocholate cotransporting polypeptide (NTCP). In addition to their anti-HBV activity, these molecules also inhibit cellular entry by the related hepatitis D virus (HDV), and are active against diverse strains of HBV (including clinically relevant nucleos(t)ide analog-resistant and vaccine escaping strains). Importantly, these macrocyclic peptides, in contrast to other NTCP-binding HBV entry inhibitors, exhibited no inhibition of NTCP-mediated bile acid uptake, making them appealing candidates for therapeutic development.

Development of a cell-based assay to identify hepatitis B virus entry inhibitors targeting the sodium taurocholate cotransporting polypeptide

Sodium taurocholate cotransporting polypeptide (NTCP) is a major entry receptor of hepatitis B virus (HBV) and one of the most attractive targets for anti-HBV drugs. We developed a cell-mediated drug screening method to monitor NTCP expression on the cell surface by generating a HepG2 cell line with tetracycline-inducible expression of NTCP and a monoclonal antibody that specifically detects cell-surface NTCP. Using this system, we screened a small molecule library for compounds that protected against HBV infection by targeting NTCP. We found that glabridin, a licorice-derived isoflavane, could suppress viral infection by inducing caveolar endocytosis of cell-surface NTCP with an IC₅₀ of ~40 μM. We also found that glabridin could attenuate the inhibitory effect of taurocholate on type I interferon signaling by depleting the level of cell-surface NTCP. These results demonstrate that our screening system could be a powerful tool for discovering drugs targeting HBV entry.

The polymorphism L204F affects transport and membrane expression of the sodium-dependent organic anion transporter SOAT (SLC10A6)

Sodium-dependent organic anion transporter (SOAT) represents a membrane transporter specific for sulfated steroid hormones, which are supposed to participate in the regulation of reproductive processes. In man, SOAT shows predominant mRNA expression in the testis and here was localized to primary spermatocytes. SOAT mRNA expression is significantly downregulated in different disorders of spermatogenesis, including hypospermatogenesis. The resulting decline of SOAT-mediated transport of sulfated steroids may participate in the impairment of functional spermatogenesis. Apart from downregulation of SOAT mRNA expression, genetic polymorphisms affecting the transport function of SOAT may have the same negative effect on spermatogenesis. Therefore, in the present study we searched for functionally relevant SOAT polymorphisms, aiming to comparatively analyze their occurrence in patients with impaired spermatogenesis vs. patients with intact spermatogenesis. We found that the SOAT polymorphism L204F showed a significantly reduced transport function for DHEAS when expressed in HEK293 cells. Although the K_m value was identical with that of the SOAT wildtype, the V_max value dramatically declined for the SOAT-L204F variant (942.5 vs. 313.6pmol×mg protein^-1×min^-1). Although the same amount of total SOAT-L204F protein was detected in transfected HEK293 cells compared to the SOAT wildtype, plasma membrane expression was significantly reduced, which points to a plasma membrane sorting defect of the SOAT-L204F variant. Groups of 20 subjects with normal spermatogenesis and 26 subjects with hypospermatogenesis were genotyped for this polymorphism. Both groups showed nearly identical distributions of the SOAT-L204F polymorphism (∼10% heterozygous and ∼5% homozygous), indicating that this polymorphism seems not be causative for hypospermatogenesis.

[Clinical and genetic analysis of a pediatric patient with sodium taurocholate cotransporting polypeptide deficiency]

Sodium taurocholate cotransporting polypeptide (NTCP) deficiency is an inborn error of bile acid metabolism caused by mutations of SLC10A1 gene. This paper reports the clinical and genetic features of a patient with this disease. A 3.3-month-old male infant was referred to the hospital with the complaint of jaundiced skin and sclera over 3 months. Physical examination revealed moderate jaundice of the skin and sclera. The liver was palpable 3.5 cm below the right subcostal margin with a medium texture. Serum biochemistry analysis revealed markedly elevated bilirubin (predominantly direct bilirubin) and total bile acids (TBA), as well as decreased 25-OH-VitD level. On pathological analysis of the biopsied liver tissue, hepatocyte ballooning and cholestatic multinucleate giant cells were noted. The lobular architecture was distorted. Infiltration of inflammatory cells, predominantly lymphocytes, was seen in the portal tracts. In response to the anti-inflammatory and liver protective drugs as well as fat-soluble vitamins over 2 months, the bilirubin and transaminases levels were improved markedly while the TBA kept elevated. Because of persisting hypercholanemia on the follow-up, SLC10A1 gene analysis was performed at his age of 17.2 months. The child proved to be a homozygote of the reportedly pathogenic variant c.800C>T (p. Ser267Phe), while the parents were both carriers. NTCP deficiency was thus diagnosed. The infant was followed up until 34.3 months old. He developed well in terms of the anthropometric indices and neurobehavioral milestones. The jaundice disappeared completely. The liver size, texture and function indices all recovered. However, the hypercholanemia persisted, and the long-term outcome needs to be observed.

Age-dependent glycosylation of the sodium taurocholate cotransporter polypeptide: From fetal to adult human livers

Sodium taurocholate cotransporter polypeptide (NTCP), mainly expressed on the sinusoidal membrane of hepatocytes, is one of the major transporters responsible for liver bile acid (BA) re-uptake. NTCP transports conjugated BA from the blood into hepatocytes and is crucial for correct enterohepatic circulation. Studies have shown that insufficient hepatic clearance of BA correlates with elevated serum BA in infants younger than 1 year of age. In the current study, we investigated human NTCP messenger RNA and protein expression by using reverse-transcription quantitative polymerase chain reaction and immunoblotting in isolated and cryopreserved human hepatocytes from two different age groups, below and above 1 year of age. Here, we show that NTCP messenger RNA expression is not modulated whereas NTCP protein posttranslational glycosylation is modulated in an age-dependent manner. These results were confirmed by quantification analysis of NTCP 55-kDa N-glycosylated bands, which showed significantly less total NTCP protein in donors below 1 year of age compared to donors older than 1 year. NTCP tissue localization was also analyzed by means of immunofluorescence. This revealed that NTCP cellular localization in fetal samples was mainly perinuclear, suggesting that NTCP is not glycosylated, while its postnatal localization on the plasma membrane is age dependent compared to multidrug resistant protein 2, which is apical starting in fetal life. Conclusion: After birth, the NTCP age-dependent maturation process requires approximately 1 year to complete NTCP glycosylation in human hepatocytes. Therefore, NTCP late posttranslational glycosylation appears to be important for correct NTCP membrane localization, which might explain physiologic cholestasis in neonatal life and might play a central role for HBV infection after birth. (Hepatology Communications 2018;2:693-702).

Monozygotic Twins Suffering From Sodium Taurocholate Cotransporting Polypeptide Deficiency: A Case Report

Sodium taurocholate cotransporting polypeptide (NTCP) is a carrier protein encoded by the human SLC10A1 gene, acting as the principal transporter of conjugated bile salts from the plasma into hepatocytes. Although NTCP was cloned as early as in 1994 and its function has been studied extensively, clinical description of NTCP deficiency remains rather limited thus far. The patients in this paper were 2 female monozygotic twins, who were referred to our hospital at the age 2 years with the complaint of persistently-raised total bile acids (TBA) for 21 months. At age 3 months, they were both diagnosed to have cholestatic liver disease due to raised serum TBA and direct bilirubin (DBIL) with the fraction >20% of the elevated total bilirubin (TBIL). Thereafter, their jaundice subsided and the DBIL levels recovered gradually, while serum TBA remained raised persistently. In view of their refractory hypercholanemia but negative symptoms and signs, SLC10A1 genetic analysis was performed for all family members to evaluate the possibility of NTCP deficiency. As a result, the twins were both homozygotes, while the parents, carriers, of the reportedly pathogenic variant c.800C>T (p.Ser267Phe). These findings suggested that NTCP deficiency may be a unique genetic factor causing transient cholestasis in early infancy, as well as, persistent hypercholanemia in pediatric patients.