Kinetics of the bile acid transporter and hepatitis B virus receptor Na+/taurocholate cotransporting polypeptide (NTCP) in hepatocytes

A new class of hepatitis B and D virus entry inhibitors, proanthocyanidin and its analogs, that directly act on the viral large surface proteins

Introduction of direct-acting antivirals against hepatitis C virus (HCV) has provided a revolutionary improvement in the treatment outcome. In contrast to HCV, however, the strategy for developing new antiviral agents against hepatitis B virus (HBV), especially viral-targeting compounds, is limited because HBV requires only four viral genes for its efficient replication/infection. Here, we identify an oligomeric flavonoid, proanthocyanidin (PAC) and its analogs, which inhibit HBV entry into host cells by targeting the HBV large surface protein (LHBs). Through cell-based chemical screening, PAC was identified to inhibit HBV infection with little cytotoxic effect. PAC prevented the attachment of the preS1 region in the LHBs to its cellular receptor, sodium taurocholate cotransporting polypeptide (NTCP). PAC was shown to target HBV particles and impair their infectivity, whereas it did not affect the NTCP-mediated bile acid transport activity. Chemical biological techniques demonstrated that PAC directly interacted with the region essential for receptor binding in the preS1 region in the LHBs protein. Importantly, PAC had a pan-genotypic anti-HBV activity and was also effective against a clinically relevant nucleoside analog-resistant HBV isolate. We further showed that PAC augmented the ability of a nucleoside analog, tenofovir, to interrupt HBV spread over time in primary human hepatocytes by cotreatment. Moreover, derivative analysis could identify small molecules that demonstrated more-potent anti-HBV activity over PAC.

CONCLUSION

PAC and its analogs represent a new class of anti-HBV agents that directly target the preS1 region of the HBV large surface protein. These agents could contribute to the development of a potent, well-tolerated, and broadly active inhibitor of HBV infection. (Hepatology 2017;65:1104-1116).

Human stem cell-derived hepatocytes as a model for hepatitis B virus infection, spreading and virus-host interactions

BACKGROUND & AIMS

One major obstacle of hepatitis B virus (HBV) research is the lack of efficient cell culture system permissive for viral infection and replication. The aim of our study was to establish a robust HBV infection model by using hepatocyte-like cells (HLCs) derived from human pluripotent stem cells.

METHODS

HLCs were differentiated from human embryonic stem cells and induced pluripotent stem cells. Maturation of hepatocyte functions was determined. After HBV infection, total viral DNA, cccDNA, total viral RNA, pgRNA, HBeAg and HBsAg were measured.

RESULTS

More than 90% of the HLCs expressed strong signals of human hepatocyte markers, like albumin, as well as known host factors required for HBV infection, suggesting that these cells possessed key features of mature hepatocytes. Notably, HLCs expressed the viral receptor sodium-taurocholate cotransporting polypeptide more stably than primary human hepatocytes (PHHs). HLCs supported robust infection and some spreading of HBV. Finally, by using this model, we identified two host-targeting agents, genistin and PA452, as novel antivirals.

CONCLUSIONS

Stem cell-derived HLCs fully support HBV infection. This novel HLC HBV infection model offers a unique opportunity to advance our understanding of the molecular details of the HBV life cycle; to further characterize virus-host interactions and to define new targets for HBV curative treatment.

LAY SUMMARY

Our study used human pluripotent stem cells to develop hepatocyte-like cells (HLCs) capable of expressing hepatocyte markers and host factors important for HBV infection. These cells fully support HBV infection and virus-host interactions, allowing for the identification of two novel antiviral agents. Thus, stem cell-derived HLCs provide a highly physiologically relevant system to advance our understanding of viral life cycle and provide a new tool for antiviral drug screening and development.

N-Glycosylation of the Na+-Taurocholate Cotransporting Polypeptide (NTCP) Determines Its Trafficking and Stability and Is Required for Hepatitis B Virus Infection

The sodium/bile acid cotransporter NTCP was recently identified as a receptor for hepatitis B virus (HBV). NTCP is glycosylated and the role of glycans in protein trafficking or viral receptor activity is not known. NTCP contains two N-linked glycosylation sites and asparagine amino acid residues N5 and N11 were mutated to a glutamine to generate NTCP with a single glycan (NTCP-N5Q or NTCP- N11Q) or no glycans (NTCP- N5,11Q). HepG2 cells expressing NTCP with a single glycan supported HBV infection at a comparable level to NTCP-WT. The physiological function of NTCP, the uptake of bile acids, was also not affected in cells expressing these single glycosylation variants, consistent with their trafficking to the plasma membrane. However, glycosylation-deficient NTCP (NTCP-N5,11Q) failed to support HBV infection, showed minimal cellular expression and was degraded in the lysosome. This affected the physiological bile acid transporter function of NTCP-N5,11Q in a similar fashion. In conclusion, N-glycosylation is required for efficient NTCP localization at the plasma membrane and subsequent HBV infection and these characteristics are preserved in NTCP carrying a single carbohydrate moiety.

Live Cell Imaging Confocal Microscopy Analysis of HBV Myr-PreS1 Peptide Binding and Uptake in NTCP-GFP Expressing HepG2 Cells

To obtain basic knowledge about specific molecular mechanisms involved in the entry of pathogens into cells is the basis for establishing pharmacologic substances blocking initial viral binding, infection, and subsequent viral spread. Lack of information about key cellular factors involved in the initial steps of HBV infection has hampered the characterization of HBV binding and entry for decades. However, recently, the liver-specific sodium-dependent taurocholate cotransporting polypeptide (NTCP) has been discovered as a functional receptor for HBV and HDV, thus opening the field for new concepts of basic binding and entry of HBV and HDV. Here, we describe practical issues of a basic in vitro assay system to examine kinetics and mechanisms of receptor-dependent HBV binding, uptake, and intracellular trafficking by live-cell imaging confocal microscopy. The assay system is comprised of HepG2 cells expressing a NTCP-GFP fusion-protein and chemically synthesized, fluorophore-labeled part of HBV surface protein, spanning the first N-terminal 48 amino acids of preS1 of the large hepatitis B virus surface protein.

DNA Polymerase κ Is a Key Cellular Factor for the Formation of Covalently Closed Circular DNA of Hepatitis B Virus

Hepatitis B virus (HBV) infection of hepatocytes begins by binding to its cellular receptor sodium taurocholate cotransporting polypeptide (NTCP), followed by the internalization of viral nucleocapsid into the cytoplasm. The viral relaxed circular (rc) DNA genome in nucleocapsid is transported into the nucleus and converted into covalently closed circular (ccc) DNA to serve as a viral persistence reservoir that is refractory to current antiviral therapies. Host DNA repair enzymes have been speculated to catalyze the conversion of rcDNA to cccDNA, however, the DNA polymerase(s) that fills the gap in the plus strand of rcDNA remains to be determined. Here we conducted targeted genetic screening in combination with chemical inhibition to identify the cellular DNA polymerase(s) responsible for cccDNA formation, and exploited recombinant HBV with capsid coding deficiency which infects HepG2-NTCP cells with similar efficiency of wild-type HBV to assure cccDNA synthesis is exclusively from de novo HBV infection. We found that DNA polymerase κ (POLK), a Y-family DNA polymerase with maximum activity in non-dividing cells, substantially contributes to cccDNA formation during de novo HBV infection. Depleting gene expression of POLK in HepG2-NTCP cells by either siRNA knockdown or CRISPR/Cas9 knockout inhibited the conversion of rcDNA into cccDNA, while the diminished cccDNA formation in, and hence the viral infection of, the knockout cells could be effectively rescued by ectopic expression of POLK. These studies revealed that POLK is a crucial host factor required for cccDNA formation during a de novo HBV infection and suggest that POLK may be a potential target for developing antivirals against HBV.

HBV chronically infects 240 million people worldwide. Persistent HBV infection relies on stable maintenance of the nuclear form of viral genome, the covalently closed circular (ccc) DNA. However, the molecular mechanism underlying the conversion of HBV genomic relaxed circular (rc) DNA into cccDNA remains elusive. Our studies reported herein provide unambiguous evidence suggesting that host DNA polymerase κ (POLK) is required for repairing the gap of rcDNA and formation of cccDNA in a de novo HBV infection. POLK is thus a potential therapeutic target for treatment of chronic hepatitis B.

Prediction of HBF-0259 interactions with hepatitis B Virus receptors and surface antigen secretory factors

Hepatitis B virus (HBV) is an etiological agent of viral hepatitis, which may lead to cirrhosis, and hepatocellular carcinoma. Current treatment strategies have not shown promising effect to date but various complications such as, drug toxicity-resistance have been reported. Study on newly discovered compounds, with minimal side effects, as specific HBV inhibitors is a fundamental subject introducing new biologic drugs. Here, we aimed to, by prediction, estimate interactions of HBF-0259 as a non-toxic anti-HBV compound on inhibiting the HBV through either interaction with the viral entry or HBsAg secreting factors using In Silico procedure. Molecular docking was performed by Hex 8.0.0 software to predict the interaction energy (Etot) between HBF-0259 and known cellular factors involved in HBV entry and HBsAg secreting factors. Hex 8.0.0 also employed to create protein-protein complexes. These interactions were then used to analyze the binding site of HBF-0259 within the assumed receptors by MGLTools software. Finally, the amino acid sequences involved in this interaction were aligned for any conservancy. Here, we showed that HBF-0259 Etot with CypA (-545.41 kcal/mol) and SCCA1 (499.68 kcal/mol), involved in HBsAg secretion and HBV integration, respectively, was higher than other interactions. Furthermore, HBF-0259 predicted interaction energy was even higher than those of CypA inhibitors. In addition, we claim that preS1 and/or preS2 regions within HBsAg are not suitable targets for HBF-0259. HBF-0259 has higher interaction energy with CypA and SCCA1, even more than other known receptors, co-receptors, viral ligands, and secretory factors. HBF-0259 could be introduced as potent anti-viral compound in which CypA and or SCCA1, as previously shown, are involved.

Unusual Features of Sodium Taurocholate Cotransporting Polypeptide as a Hepatitis B Virus Receptor

Cell culture (cc)-derived hepatitis B virus (HBV) can infect differentiated HepaRG cells, but efficient infection requires addition of polyethylene glycol (PEG) during inoculation. Identification of sodium taurocholate cotransporting polypeptide (NTCP) as an HBV receptor enabled ccHBV infection of NTCP reconstituted HepG2 cells, although very little hepatitis B surface antigen (HBsAg) is produced. We found infection by patient serum-derived HBV (sHBV), which required purification of viral particles through ultracentrifugation or PEG precipitation, was PEG independent and much more efficient in HepaRG cells than in HepG2/NTCP cells. In contrast to hepatitis B e antigen (HBeAg), HBsAg was not a reliable marker of productive sHBV infection at early time points. A low HBsAg/HBeAg ratio by ccHBV-infected HepG2/NTCP cells was attributable to dimethyl sulfoxide (DMSO) in culture medium, NTCP overexpression, and HBV genotype D. HepG2/NTCP cells released more viral antigens than HepG2 cells after HBV genome delivery by adeno-associated virus, and stable expression of NTCP in a ccHBV producing cell line increased viral mRNAs, proteins, replicative DNA, and covalently closed circular DNA. NTCP protein expression in HepG2/NTCP cells, despite being driven by the cytomegalovirus promoter, was markedly increased by DMSO treatment. This at least partly explains ability of DMSO to promote ccHBV infection in such cell lines. In conclusion, NTCP appeared inefficient to mediate infection by serum-derived HBV. It could promote HBV RNA transcription while inhibiting HBsAg secretion. Efficient PEG-independent sHBV infection of HepaRG cells permits comparative studies of diverse clinical HBV isolates and will help identify additional factors on virion surface promoting attachment to hepatocytes.

IMPORTANCE

Currently in vitro infection with hepatitis B virus (HBV) depends on cell culture-derived HBV inoculated in the presence of polyethylene glycol. We found patient serum-derived HBV could efficiently infect differentiated HepaRG cells independent of polyethylene glycol, which represents a more physiological infection system. Serum-derived HBV has poor infectivity in HepG2 cells reconstituted with sodium taurocholate cotransporting polypeptide (NTCP), the currently accepted HBV receptor. Moreover, HepG2/NTCP cells secreted very little hepatitis B surface antigen after infection with cell culture-derived HBV, which was attributed to NTCP overexpression, genotype D virus, and dimethyl sulfoxide added to culture medium. NTCP could promote HBV RNA transcription, protein expression, and DNA replication in HepG2 cells stably transfected with HBV DNA, while dimethyl sulfoxide could increase NTCP protein level despite transcriptional control by a cytomegalovirus promoter. Therefore, this study revealed several unusual features of NTCP as an HBV receptor and established conditions for efficient serum virus infection in vitro.

Visualization of hepatitis B virus entry - novel tools and approaches to directly follow virus entry into hepatocytes

Hepatitis B virus (HBV) is a widespread human pathogen, responsible for chronic infections of ca. 240 million people worldwide. Until recently, the entry pathway of HBV into hepatocytes was only partially understood. The identification of human sodium taurocholate cotransporting polypeptide (NTCP) as a bona fide receptor of HBV has provided us with new tools to investigate this pathway in more details. Combined with advances in virus visualization techniques, approaches to directly visualize HBV cell attachment, NTCP interaction, virion internalization and intracellular transport are now becoming feasible. This review summarizes our current understanding of how HBV specifically enters hepatocytes, and describes possible visualization strategies applicable for a deeper understanding of the underlying cell biological processes.

Hepatitis B Virus Infection of a Mouse Hepatic Cell Line Reconstituted with Human Sodium Taurocholate Cotransporting Polypeptide

Hepatitis B virus (HBV) enters hepatocytes via its receptor, human sodium taurocholate cotransporting polypeptide (hNTCP). So far, HBV infection has been achieved only in human hepatic cells reconstituted with hNTCP and not in cells of mouse origin. Here, the first mouse liver cell line (AML12) which gains susceptibility to HBV upon hNTCP expression is described. Thus, HBV infection of receptor-expressing mouse hepatocytes does not principally require a human cofactor but can be triggered by endogenous murine determinants.

Entry of hepatitis B and hepatitis D virus into hepatocytes: Basic insights and clinical implications

For almost three decades following the discovery of the human Hepatitis B Virus (HBV) the early events of virus infection (attachment to hepatocytes, specific binding to a receptor on hepatocytes) remained enigmatic. The gradual improvement of tissue culture systems for HBV has enabled the identification of viral determinants for viral infectivity and facilitated the discovery of the human sodium taurocholate co-transporting polypeptide (hNTCP) as a liver specific receptor of HBV and its satellite, the human Hepatitis Delta Virus (HDV). These findings are currently leading basic and clinical research activities in new directions. (1) Stable hNTCP-expressing cell lines have become a valuable platform to study the full HBV replication cycle from its native template, the cccDNA. (2) The suitability of NTCP complemented cell culture systems for high throughput screening approaches will facilitate identification of novel host factors involved in HBV replication (including those that determine the peculiar host specificity of HBV infection) and will enable identification and development of novel drug candidates for improved therapeutics. (3) Since NTCP is a major host-specific restriction factor for HBV and HDV, hNTCP-expressing animals provide the basis for future susceptible in vivo models. (4) The concept obtained with the entry inhibitor Myrcludex B demonstrates that NTCP is a suitable target for clinical interference with viral entry. This will foster further clinical approaches aiming at curative combination therapies.

Evidence that hepatitis B virus replication in mouse cells is limited by the lack of a host cell dependency factor

BACKGROUND & AIMS

Hepatitis B virus (HBV) is a major human pathogen restricted to hepatocytes. Expression of the specific receptor human sodium taurocholate cotransporting polypeptide (hNTCP) in mouse hepatocytes renders them susceptible to hepatitis delta virus (HDV), a satellite of HBV; however, HBV remains restricted at an early stage of replication. This study aims at clarifying whether this restriction is caused by the lack of a dependency factor or the activity of a restriction factor.

METHODS

Six hNTCP-expressing mouse and human cell lines were generated and functionally characterized. By fusion with replication-supporting but non-infectable HepG2 cells, we analysed the ability of these heterokaryonic cells to fully support HBV replication by HBcAg expression and HBsAg/HBeAg secretion.

RESULTS

While hNTCP expression in three mouse cell lines and the non-hepatic human HeLa cells conferred susceptibility to HDV, HBV replication was still restricted. Upon fusion of refractive cells to HepG2 cells, all heterokaryonic cells supported receptor-mediated infection with HBV. hNTCP was provided by the mouse cells and replication competence came from the HepG2 cell line. Transfection of a covalently closed circular DNA (cccDNA)-like molecule into non-susceptible cells promoted gene expression, indicating that the limiting step is upstream of cccDNA formation.

CONCLUSIONS

In addition to the expression of hNTCP, establishment of HBV infection in mouse and non-hepatocytic human cell lines requires supplementation with a dependency factor and is not limited by a restriction factor. This result opens new avenues for the development of a fully permissive immunocompetent HBV mouse model.

Recent developments in antivirals against hepatitis B virus

Chronic hepatitis B virus (HBV) infection (CHB) is a major cause of cirrhosis and hepatocellular carcinoma (HCC). Although the availability of HBV vaccines effectively reduces the incidence of HBV infection, the healthcare burden from CHB remains high. Several antiviral agents, such as (pegylated-) interferon-α and nucleos(t)ide analogs are approved by US FDA for chronic HBV infection management. Entecavir (ETV) and tenofovir disoproxil fumarate (TDF) have been recommended as the first-line anti-HBV drugs for excellent viral suppression with a low risk of antiviral resistance, but the cost and need for essentially life-long treatment are considerable challenges. And none of these current treatments can eradicate the intracellular virus. Given these issues, there is still an unmet medical need for an efficient HBV cure. We summarize here the key developments of antivirals against hepatitis B virus, including HBV replication cycle inhibitors and host immune regulators.

Hepatitis B virus efficiently infects non-adherent hepatoma cells via human sodium taurocholate cotransporting polypeptide

Sodium taurocholate cotransporting polypeptide (NTCP) has been reported as a functional receptor for hepatitis B virus (HBV) infection. However, HBV could not efficiently infect HepG2 cells expressing NTCP (NTCP-HepG2 cells) under adherent monolayer-cell conditions. In this study, NTCP was mainly detected in the basolateral membrane region, but not the apical site, of monolayer NTCP-HepG2 cells. We hypothesized that non-adherent cell conditions of infection would enhance HBV infectivity. Non-adherent NTCP-HepG2 cells were prepared by treatment with trypsin and EDTA, which did not degrade NTCP in the membrane fraction. HBV successfully infected NTCP-HepG2 cells at a viral dose 10 times lower in non-adherent phase than in adherent phase. Efficient infection of non-adherent NTCP-HepG2 cells with blood-borne or cell-culture-derived HBV was observed and was remarkably impaired in the presence of the myristoylated preS1 peptide. HBV could also efficiently infect HepaRG cells under non-adherent cell conditions. We screened several compounds using our culture system and identified proscillaridin A as a potent anti-HBV agent with an IC₅₀ value of 7.2 nM. In conclusion, non-adherent host cell conditions of infection augmented HBV infectivity in an NTCP-dependent manner, thus providing a novel strategy to identify anti-HBV drugs and investigate the mechanism of HBV infection.

The hepatitis B virus receptor

Hepatitis B virus (HBV) infection affects 240 million people worldwide. A liver-specific bile acid transporter named the sodium taurocholate cotransporting polypeptide (NTCP) has been identified as the cellular receptor for HBV and its satellite, the hepatitis D virus (HDV). NTCP likely acts as a major determinant for the liver tropism and species specificity of HBV and HDV at the entry level. NTCP-mediated HBV entry interferes with bile acid transport in cell cultures and has been linked with alterations in bile acid and cholesterol metabolism in vivo. The human liver carcinoma cell line HepG2, complemented with NTCP, now provides a valuable platform for studying the basic biology of the viruses and developing treatments for HBV infection. This review summarizes critical findings regarding NTCP's role as a viral receptor for HBV and HDV and discusses important questions that remain unanswered.

Targeting Viral Entry for Treatment of Hepatitis B and C Virus Infections

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infections remain major health problems worldwide, with 400-500 million chronically infected people worldwide. Chronic infection results in liver cirrhosis and hepatocellular carcinoma, the second leading cause of cancer death. Current treatments for HBV limit viral replication without efficiently curing infection. HCV treatment has markedly progressed with the licensing of direct-acting antivirals (DAAs) for HCV cure, yet limited access for the majority of patients is a major challenge. Preventative and curative treatment strategies, aimed at novel targets, are needed for both viruses. Viral entry represents one such target, although detailed knowledge of the entry mechanisms is a prerequisite. For HBV, the recent discovery of the NTCP cell entry factor enabled the establishment of an HBV cell culture model and showed that cyclosporin A and Myrcludex B are NTCP-targeting entry inhibitors. Advances in the understanding of HCV entry revealed it to be a complex process involving many factors, offering several antiviral targets. These include viral envelope proteins E1 and E2, virion-associated lipoprotein ApoE, and cellular factors CD81, SRBI, EGFR, claudin-1, occludin, and the cholesterol transporter NPC1L1. Small molecules targeting SR-BI, EGFR, and NPC1L1 have entered clinical trials, whereas other viral- and host-targeted small molecules, peptides, and antibodies show promise in preclinical models. This review summarizes the current understanding of HBV and HCV entry and describes novel antiviral targets and compounds in different stages of clinical development. Overall, proof-of-concept studies indicate that entry inhibitors are a promising class of antivirals to prevent and treat HBV and HCV infections.

Host-targeting agents for treatment of hepatitis B virus infection

Hepatitis B virus (HBV) infection is a major cause of chronic liver disease, including liver cirrhosis, liver failure and hepatocellular carcinoma (HCC)-the second leading and fastest rising cause of cancer death world-wide. While de novo infection can be efficiently prevented by vaccination and chronic infection can be controlled using antivirals targeting the viral polymerase, the development of efficient antiviral strategies to eliminate the virus and thus to cure infection remains a key unmet medical need. The recent progress in the development of robust infectious HBV cell culture models now enables the investigation of the full viral life cycle, including a more detailed study of the molecular mechanisms of virus-host interactions responsible for viral persistence. The understanding of these virus-host interactions will be instrumental for the development of curative treatments. Host-dependency factors have recently emerged as promising candidates to treat and prevent infection by various pathogens. This review focuses on the potential of host-targeting agents (HTAs) as novel antivirals to treat and cure HBV infection. These include HTAs that inhibit de novo and re-infection, synthesis and spread of cccDNA as well as development of immune-based approaches eliminating or curing infected hepatocytes, including the eradication of viral cccDNA.

Sodium/taurocholate cotransporting polypeptide expression in hepatocellular carcinoma in rats

AIM: To detect the expression of sodium/taurocholate cotransporting polypeptide (Ntcp) in hepatocellular carcinoma in rats.

METHODS: Sixty Wistar rats were randomly divided into a control group and an experimental group, with 30 rats in each group. The control group was fed an ordinary diet, and the experimental group was fed diethylnitrosamine for 20 wk to induce primary liver cancer. After successful induction, liver tissues were taken to detect the expression of Ntcp protein by immunohistochemistry and Western blot.

RESULTS: Both immunohistochemistry and Western blot analysis showed that the expression of Ntcp protein was significantly higher in the experiment group than in the control group (P < 0.05).

CONCLUSION: The expression of Ntcp protein increases significantly in primary liver cancer in rats, which suggests that drugs targeting Ntcp may provide a new treatment method for primary liver cancer.

Na+/taurocholate cotransporting polypeptide expression in liver tissue of hyperlipidemia rats

AIM: To establish a rat model of hyperlipidemia, detect the expression of Na⁺/taurocholate cotransporting polypeptide (Ntcp) in this model, and explore the role of Ntcp in bile acid and cholesterol metabolism in hyperlipidemia rats.

METHODS: Sixty Wistar rats were randomly divided into two groups, a control group (control group) that was fed an ordinary diet, and a high fat diet group (experimental group) that was fed a high fat diet. Regular detection of cholesterol and bile acid content was conducted to assess whether the hyperlipidemia model was successfully established. After successful induction of hyperlipidemia, liver tissues were taken to detect the mRNA expression of Ntcp by reverse transcription-polymerase chain reaction (RT-PCR) and protein expression by immunohistochemistry (streptavidin-peroxidase).

RESULTS: In the experiment group the contents of cholesterol and bile acid were significantly higher than those in the control group. RT-PCR analysis showed that the mRNA expression of Ntcp in the liver tissue was significantly lower in the experimental group than in the control group. Immunohistochemistry showed that in the experimental group, the positive expression rate of Ntcp was 23.6%, significantly lower than 75.2% in the control group (χ² = 9.858, P < 0.05).

CONCLUSION: Ntcp expression decreases significantly in rats with hyperlipidemia, suggesting that Ntcp may be used as a target for treatment of hyperlipidemia and related diseases.

Applications of human hepatitis B virus preS domain in bio- and nanotechnology

Human hepatitis B virus (HBV) is a member of the family Hepadnaviridae, and causes acute and chronic infections of the liver. The hepatitis B surface antigen (HBsAg) contains the large (L), middle (M), and small (S) surface proteins. The L protein consists of the S protein, preS1, and preS2. In HBsAg, the preS domain (preS1 + preS2) plays a key role in the infection of hepatocytic cells by HBV and has several immunogenic epitopes. Based on these characteristics of preS, several preS-based diagnostic and therapeutic materials and systems have been developed. PreS1-specific monoclonal antibodies (e.g., MA18/7 and KR127) can be used to inhibit HBV infection. A myristoylated preS1 peptide (amino acids 2-48) also inhibits the attachment of HBV to HepaRG cells, primary human hepatocytes, and primary tupaia hepatocytes. Antibodies and antigens related to the components of HBsAg, preS (preS1 + preS2), or preS1 can be available as diagnostic markers of acute and chronic HBV infections. Hepatocyte-targeting delivery systems for therapeutic molecules (drugs, genes, or proteins) are very important for increasing the clinical efficacy of these molecules and in reducing their adverse effects on other organs. The selective delivery of diagnostic molecules to target hepatocytic cells can also improve the efficiency of diagnosis. In addition to the full-length HBV vector, preS (preS1 + preS2), preS1, and preS1-derived fragments can be useful in hepatocyte-specific targeting. In this review, we discuss the literature concerning the applications of the HBV preS domain in bio- and nanotechnology.

Sodium taurocholate cotransporting polypeptide acts as a receptor for hepatitis B and D virus

Infection of hepatitis B virus (HBV) remains a major public health problem worldwide. Understanding the viral infection and developing antivirals against HBV have been hampered by the lack of convenient culture systems and animal models for the infection. Sodium taurocholate cotransporting polypeptide (NTCP), a key bile acid transporter expressed in liver, was recently identified as a critical receptor for viral entry of HBV and its satellite virus hepatitis D virus (HDV). This finding enabled a reliable cell culture system for the viruses. Detailed studies have shown that NTCP is the major determinant for the species specificity of HBV and HDV at entry level. NTCP is responsible for most sodium-dependent bile salt uptake in liver. The molecular determinant critical for HBV/HDV infection overlaps with that for bile acids transporting on NTCP. We evaluated bile acids as potential antivirals for HBV and HDV infection, and developed bile acid derivatives that effectively block taurocholate transporting as well as viral infections. The discovery that NTCP acts as a receptor for HBV has opens a new door for future studies towards the ultimate goal of curative treatment of HBV infection.

The FDA-approved drug irbesartan inhibits HBV-infection in HepG2 cells stably expressing sodium taurocholate co-transporting polypeptide

BACKGROUND

Little is known about the early steps of the HBV life cycle due to the lack of susceptible cells permissive for viral infection. Hence, viral entry has not been exploited for antiviral targets, but the recent seminal discovery of sodium taurocholate co-transporting polypeptide (NTCP) as the cellular receptor for HBV entry opened up many avenues of investigation, making HBV entry amenable to therapeutic intervention.

METHODS

In order to exploit HBV entry, we established a HepG2-NTCP cell line that supports HBV infection. Over 70% of cells were infected at a dose of 10(4) genome equivalents (GEq) per cell. Several FDA-approved drugs with NTCP-inhibiting activity were tested for their ability to inhibit HBV infection of the cell line.

RESULTS

Consistent with their NTCP inhibitory activities, our results showed that several of them inhibit HBV infection. In particular, irbesartan, a drug used for the treatment of hypertension, inhibits HBV infection at the 50% effective concentration value of 35 μM.

CONCLUSIONS

The observation that the pharmacological inhibitors of the NTCP transporter could block HBV entry suggests that NTCP represents an attractive molecular target for therapeutic intervention in HBV infection.

Primary biliary acids inhibit hepatitis D virus (HDV) entry into human hepatoma cells expressing the sodium-taurocholate cotransporting polypeptide (NTCP)

BACKGROUND

The sodium-taurocholate cotransporting polypeptide (NTCP) is both a key bile acid (BA) transporter mediating uptake of BA into hepatocytes and an essential receptor for hepatitis B virus (HBV) and hepatitis D virus (HDV). In this study we aimed to characterize to what extent and through what mechanism BA affect HDV cell entry.

METHODS

HuH-7 cells stably expressing NTCP (HuH-7/NTCP) and primary human hepatocytes (PHH) were infected with in vitro generated HDV particles. Infectivity in the absence or presence of compounds was assessed using immunofluorescence staining for HDV antigen, standard 50% tissue culture infectious dose (TCID50) assays and quantitative PCR.

RESULTS

Addition of primary conjugated and unconjugated BA resulted in a dose dependent reduction in the number of infected cells while secondary, tertiary and synthetic BA had a lesser effect. This effect was observed both in HuH-7/NTCP and in PHH. Other replication cycle steps such as replication and particle assembly and release were unaffected. Moreover, inhibitory BA competed with a fragment from the large HBV envelope protein for binding to NTCP-expressing cells. Conversely, the sodium/BA-cotransporter function of NTCP seemed not to be required for HDV infection since infection was similar in the presence or absence of a sodium gradient across the plasma membrane. When chenodeoxycolic acid (15 mg per kg body weight) was administered to three chronically HDV infected individuals over a period of up to 16 days there was no change in serum HDV RNA.

CONCLUSIONS

Primary BA inhibit NTCP-mediated HDV entry into hepatocytes suggesting that modulation of the BA pool may affect HDV infection of hepatocytes.

HBV: Stowaway of NTCP

Several recent papers support the view that Na-taurocholate cotransporting polypeptide (NTCP), the sinusoidal bile acid transporter of hepatocytes, is a cell surface receptor enabling entry into these cells of the hepatitis B virus (HBV). This major advance in the understanding of the HBV life cycle paves the way to new therapeutic strategies aimed at blocking HBV entry into hepatocytes.

Contribution of Toll-like receptors to the control of hepatitis B virus infection by initiating antiviral innate responses and promoting specific adaptive immune responses

It is well accepted that adaptive immunity plays a key role in the control of hepatitis B virus (HBV) infection. In contrast, the contribution of innate immunity has only received attention in recent years. Toll-like receptors (TLRs) sense pathogen-associated molecule patterns and activate antiviral mechanisms, including intracellular antiviral pathways and the production of antiviral effector interferons (IFNs) and pro-inflammatory cytokines. Experimental results from in vitro and in vivo models have demonstrated that TLRs mediate the activation of cellular signaling pathways and the production of antiviral cytokines, resulting in a suppression of HBV replication. However, HBV infection is associated with downregulation of TLR expression on host cells and blockade of the activation of downstream signaling pathways. In primary HBV infection, TLRs may slow down HBV infection, but contribute only indirectly to viral clearance. Importantly, TLRs may modulate HBV-specific T- and B-cell responses in vivo, which are essential for the termination of HBV infection. Thus, TLR agonists are promising candidates to act as immunomodulators for the treatment of chronic HBV infection. Antiviral treatment may recover TLR expression and function in chronic HBV infection and may increase the efficacy of therapeutic approaches based on TLR activation. A combined therapeutic strategy with antiviral treatment and TLR activation could facilitate the restoration of HBV-specific immune responses and thereby, achieve viral clearance in chronically infected HBV patients.