Being the largest internal organ of the human body with the unique ability of self-regeneration, the liver is involved in a wide variety of vital functions that require highly orchestrated and controlled biochemical processes. Increasing evidence suggests that microRNAs (miRNAs) are essential for the regulation of liver development, regeneration and metabolic functions. Hence, alterations in intrahepatic miRNA networks have been associated with liver disease including hepatitis, steatosis, cirrhosis and hepatocellular carcinoma (HCC). miR-122 is the most frequent miRNA in the adult liver, and a central player in liver biology and disease. Furthermore, miR-122 has been shown to be an essential host factor for hepatitis C virus (HCV) infection and an antiviral target, complementary to the standard of care using direct-acting antivirals or interferon-based treatment. This review summarizes our current understanding of the key role of miR-122 in liver physiology and disease, highlighting its role in HCC and viral hepatitis. We also discuss the perspectives of miRNA-based therapeutic approaches for viral hepatitis and liver disease.

Hepatitis C virus (HCV) is a major cause of liver cirrhosis and hepatocellular carcinoma. Preventive modalities are absent and the current antiviral treatment is limited by resistance, toxicity, and high costs. Viral entry is required for initiation, spread, and maintenance of infection, and thus is a promising target for antiviral therapy. HCV entry is a highly orchestrated process involving viral and host cell factors. These include the viral envelope glycoproteins E1 and E2, CD81, scavenger receptor BI, and tight junction proteins claudin-1 and occludin. Recent studies in preclinical models and HCV-infected patients have demonstrated that the virus has developed multiple strategies to escape host immune responses during viral entry. These include evasion from neutralizing antibodies and viral spread by cell-cell transmission. These challenges have to be taken into account for the design of efficient antiviral strategies. Thus, a detailed understanding of the mechanisms of viral entry and escape is a prerequisite to define viral and cellular targets and develop novel preventive and therapeutic antivirals. This review summarizes the current knowledge about the molecular mechanisms of HCV entry into hepatocytes, highlights novel targets and reviews the current preclinical and clinical development of compounds targeting entry. Proof-of-concept studies suggest that HCV entry inhibitors are a novel and promising class of antivirals widening the preventive and therapeutic arsenal against HCV infection.

Hepatitis C virus (HCV) is a human hepatotropic virus, but the relevant host factors restricting HCV infection to hepatocytes are only partially understood. We demonstrate that exogenous expression of defined host factors reconstituted the entire HCV life cycle in human nonhepatic 293T cells. This study shows robust HCV entry, RNA replication, and production of infectious virus in human nonhepatic cells and highlights key host factors required for liver tropism of HCV.

Hepatitis C virus (HCV)-induced chronic liver disease is a leading cause of hepatocellular carcinoma (HCC). However, the molecular mechanisms underlying HCC development following chronic HCV infection remain poorly understood. MicroRNAs (miRNAs) play an important role in homeostasis within the liver, and deregulation of miRNAs has been associated with liver disease, including HCC. While host miRNAs are essential for HCV replication, viral infection in turn appears to induce alterations of intrahepatic miRNA networks. Although the cross talk between HCV and liver cell miRNAs most likely contributes to liver disease pathogenesis, the functional involvement of miRNAs in HCV-driven hepatocyte injury and HCC remains elusive. Here we combined a hepatocyte-like cell-based model system, high-throughput small RNA sequencing, computational analysis, and functional studies to investigate HCV-miRNA interactions that may contribute to liver disease and HCC. Profiling analyses indicated that HCV infection differentially regulated the expression of 72 miRNAs by at least 2-fold, including miRNAs that were previously described to target genes associated with inflammation, fibrosis, and cancer development. Further investigation demonstrated that the miR-146a-5p level was consistently increased in HCV-infected hepatocyte-like cells and primary human hepatocytes, as well as in liver tissue from HCV-infected patients. Genome-wide microarray and computational analyses indicated that miR-146a-5p overexpression modulates pathways that are related to liver disease and HCC development. Furthermore, we showed that miR-146a-5p has a positive impact on late steps of the viral replication cycle, thereby increasing HCV infection. Collectively, our data indicate that the HCV-induced increase in miR-146a-5p expression both promotes viral infection and is relevant for pathogenesis of liver disease. IMPORTANCE HCV is a leading cause of chronic liver disease and cancer. However, how HCV induces liver cancer remains poorly understood. There is accumulating evidence that a viral cure does not eliminate the risk for HCC development. Thus, there is an unmet medical need to develop novel approaches to predict and prevent virus-induced HCC. miRNA expression is known to be deregulated in liver disease and cancer. Furthermore, miRNAs are essential for HCV replication, and HCV infection alters miRNA expression. However, how miRNAs contribute to HCV-driven pathogenesis remains elusive. Here we show that HCV induces miRNAs that may contribute to liver injury and carcinogenesis. The miR-146a-5p level was consistently increased in different cell-based models of HCV infection and in HCV patient-derived liver tissue. Furthermore, miR-146a-5p increased HCV infection. Collectively, our data are relevant to understanding viral pathogenesis and may open perspectives for novel biomarkers and prevention of virus-induced liver disease and HCC.

Scavenger receptor class B type I (SR-BI) is a high-density lipoprotein (HDL) receptor highly expressed in the liver and modulating HDL metabolism. Hepatitis C virus (HCV) is able to directly interact with SR-BI and requires this receptor to efficiently enter into hepatocytes to establish productive infection. A complex interplay between lipoproteins, SR-BI and HCV envelope glycoproteins has been reported to take place during this process. SR-BI has been demonstrated to act during binding and postbinding steps of HCV entry. Although the SR-BI determinants involved in HCV binding have been partially characterized, the postbinding function of SR-BI remains largely unknown. To uncover the mechanistic role of SR-BI in viral initiation and dissemination, we generated a novel class of anti–SR-BI monoclonal antibodies that interfere with postbinding steps during the HCV entry process without interfering with HCV particle binding to the target cell surface. Using the novel class of antibodies and cell lines expressing murine and human SR-BI, we demonstrate that the postbinding function of SR-BI is of key impact for both initiation of HCV infection and viral dissemination. Interestingly, this postbinding function of SR-BI appears to be unrelated to HDL interaction but to be directly linked to its lipid transfer function. Conclusion: Taken together, our results uncover a crucial role of the SR-BI postbinding function for initiation and maintenance of viral HCV infection that does not require receptor-E2/HDL interactions. The dissection of the molecular mechanisms of SR-BI–mediated HCV entry opens a novel perspective for the design of entry inhibitors interfering specifically with the proviral function of SR-BI.