MicroRNA-135a Modulates Hepatitis C Virus Genome Replication through Downregulation of Host Antiviral Factors
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Cellular microRNAs (miRNAs) have been shown to modulate HCV infection via directly acting on the viral genome or indirectly through targeting the virus-associated host factors. Recently we generated a comprehensive map of HCV–miRNA interactions through genome-wide miRNA functional screens and transcriptomics analyses. Many previously unappreciated cellular miRNAs were identified to be involved in HCV infection, including miR-135a, a human cancer-related miRNA. In the present study, we investigated the role of miR-135a in regulating HCV life cycle and showed that it preferentially enhances viral genome replication. Bioinformatics-based integrative analyses and subsequent functional assays revealed three antiviral host factors, including receptor interacting serine/threonine kinase 2 (RIPK2), myeloid differentiation primary response 88 (MYD88), and C-X-C motif chemokine ligand 12 (CXCL12), as bona fide targets of miR-135a. These genes have been shown to inhibit HCV infection at the RNA replication stage. Our data demonstrated that repression of key host restriction factors mediated the proviral effect of miR-135a on HCV propagation. In addition, miR-135a hepatic abundance is upregulated by HCV infection in both cultured hepatocytes and human liver, likely mediating a more favorable environment for viral replication and possibly contributing to HCV-induced liver malignancy. These results provide novel insights into HCV–host interactions and unveil molecular pathways linking miRNA biology to HCV pathogenesis.
KeywordsHepatitis C virus (HCV) Genome replication Virus–host interactions miR-135a Antiviral factors
Hepatitis C virus (HCV), a positive-sense, single-stranded RNA virus of the Flaviviridae family, chronically infects approximately 150 million people worldwide (Mohd Hanafiah et al. 2013). Without intervention, HCV infection often leads to progressive liver damage that induces cirrhosis, liver failure, and hepatocellular carcinoma (HCC). While highly effective direct-acting antiviral (DAA) regimens have been developed, eradication of the virus does not completely eliminate the increased risk of advanced liver diseases, including fibrosis and HCC (El-Serag et al. 2016). It is therefore of medical interest to elucidate the molecular mechanisms underlying HCV-associated liver dysfunctions.
HCV depends heavily on host or cellular factors to establish persistent infection and trigger unique hepatic pathological processes (Randall et al. 2007; Li et al. 2009; Tai et al. 2009; Lupberger et al. 2011; Reiss et al. 2011; Li et al. 2014). Cellular microRNAs (miRNAs) represent such host dependencies and play important roles in regulating viral infection and related pathogenesis. MicroRNAs are 22-nucleotide non-coding RNAs that repress target gene expression and manipulate a multitude of pathophysiological processes in cells. miRNAs exert their effects by binding to predominantly the 3′ untranslated regions (UTRs) of the target mRNAs, thus suppressing their expression via mRNA degradation and/or translational repression scenarios (Bartel 2009; Fabian et al. 2010). Host miRNAs have been shown to influence the life cycle of many viruses both directly, through interactions with the viral genome, or indirectly, through regulation of critical virus-associated cellular pathways. A prime example is the reliance of productive HCV infection on the expression of a liver-specific cellular miRNA–miR-122. Mechanistically, miR-122 binds to the HCV 5′ UTR, stimulates viral RNA synthesis and translation, and protects the viral genome from degradation by the 5′ exoribonucleases Xrn1 and Xrn2 (Jopling et al. 2005; Li et al. 2013; Sedano and Sarnow 2014; Li et al. 2015; Masaki et al. 2015). Interestingly, HCV RNA may sequester miR-122 through a “sponge” effect that causes global de-repression of cellular miR-122 targets. This mechanism potentially facilitates the long-term oncogenic features of HCV infection (Luna et al. 2015, 2017). HCV infection is also modulated by multiple other host miRNAs in an intricate cellular regulatory network that engages both proviral and antiviral mechanisms (Singaravelu et al. 2014). Dysfunction and deregulation of the cellular miRNA landscape have also been attributed to HCV-associated liver diseases, including hepatic steatosis, fibrosis and liver cancer (Szabo and Bala 2013; Bandiera et al. 2015).
Recently, we interrogated the entire miRNome for their impacts on HCV infection by conducting a genome-wide miRNA functional screen. Numerous interactions between cellular miRNAs and the HCV life cycle were identified (Li et al. 2017). Among the previously unknown HCV-associated miRNAs is miR-135a, a miRNA that is elevated broadly across many cancer types and a candidate driver of HCV-associated hepatocarcinogenesis (Van Renne et al. 2018). Overexpression of miR-135a mimic in cultured hepatocytes significantly enhanced HCV core protein expression and viral RNA levels, eliciting its proviral function in modulating HCV infection (Li et al. 2017). Nevertheless, the precise role and mode of action of miR-135a in enhancing HCV infection remain uncharacterized. Moreover, whether the expression or function of miR-135a is affected by HCV and contributes to virus-associated progressive liver damage is not yet known. In this study, we investigated the effects of miR-135a on HCV life cycle and demonstrated that this miRNA modulates HCV genome replication through its actions on several critical antiviral host factors, which have been implicated in HCV-mediated liver pathogenesis.
Materials and Methods
Cells and Viruses
The human hepatoma Huh7 derivative cell line Huh7.5.1 (provided by F.V. Chisari of The Scripps Research Institute, La Jolla, CA) was maintained in complete growth medium (DMEM; Corning) containing 10% Fetal Bovine Serum (Corning). The cell line was free from mycoplasma contamination, regularly tested using a Mycoplasma Detection Kit (Thermo Fisher Scientific).
HCV genotype 2a JFH-1 strain (provided by T. Wakita of the National Institute of Infectious Diseases, Tokyo, Japan) was propagated and infectivity was titrated as previously described (Wakita et al. 2005; Kato et al. 2007). The chimeric HCV cDNA clones of multiple genotypes and subtypes (provided by J. Bukh of Copenhagen University Hospital, Copenhagen, Denmark) were JFH-1–based recombinants containing the structural proteins (core, E1, and E2), p7 and NS2 of genotypes 1a (H77C), 1b (HC-J4), 2b (HC-J8), 3a (S52), and 4a (ED43), respectively. The viral stocks were prepared and titrated as described previously (Gottwein et al. 2009). Unless otherwise indicated, HCV infection was conducted at a multiplicity of infection (MOI) of 0.5, and assays were typically performed at 48 h post-infection.
Patients and Liver Biopsies
Liver biopsy samples were obtained from both healthy volunteers and patients with chronic hepatitis C with genotype 1b infection. All patients provided written informed consent and the protocol was approved by the Institutional Review Board of the National Institute of Diabetes and Digestive and Kidney Diseases and the National Institute of Arthritis and Musculoskeletal and Skin Diseases.
In 12- or 96-well format, miRIDIAN human miRNA mimics or miR-122 hairpin inhibitor (Dharmacon) were transfected into Huh7.5.1 cells at a final concentration of 25 nmol/L, applying a reverse transfection protocol using Oligofectamine (Invitrogen). Typically, cells were incubated with miRNAs for 72 h, when miRNA-mediated gene regulation is robust, and then either further treated or harvested for various assays. miRIDIAN microRNA Mimic Negative Control #2 (Dharmacon) was used as the negative control in multiple assays.
Human ON-TARGETplus SMARTpool siRNAs (Dharmacon) were transfected into Huh7.5.1 cells at a 50 nmol/L final concentration, utilizing Oligofectamine and a reverse transfection protocol as previously described (Li et al. 2009). ON-TARGETplus Non-targeting siRNA #2 (siNT) was used as a negative control. Unless otherwise noted, at 72 h post-transfection, when silencing efficiency reaches its maximal level, cells were either harvested for gene expression analysis, or infected with HCV for further virological assays.
HCV Life Cycle Assays
To evaluate the potential impacts of miR-135a on various steps of HCV infection, viral life cycle assays were conducted. Specifically, HCVpp (for entry), subgenomic replicon (for IRES-mediated translation and/or viral genome replication), HCVsc (for single cycle infection) and HCVcc (for the entire life cycle) assays were performed. Detailed assay methods are described below.
HCV Entry Assay
HCV pseudoparticles (HCVpp) and VSV-Gpp were generated as previously described (Li et al. 2014). HCVpp harboring E1/E2 glycoproteins from genotypes 1a and 1b were derived from the plasmids pHCV7a and pHCV-E1E2.1b3 (provided by F. L. Cosset of the INSERM U412, Lyon, France), respectively, as described previously (Lavillette et al. 2005). Huh7.5.1 cells were seeded at the density of 4500 cells per well in 96-well white microplates (Greiner Bio-one), and then transfected with various miRNA mimics (at 25 nmol/L) or SMARTpool siRNAs (at 50 nmol/L) using a reverse transfection protocol (in 5 replicates). After 72 h, cells were infected with HCVpp or VSV-Gpp. At 48 h post-infection, cells were lysed in 1 × Reporter Lysis Buffer (Promega). Firefly luciferase activity was subsequently measured per manufacturer’s instructions (Promega) using a POLARstar Omega multidetection microplate reader (BMG Labtech).
HCV Subgenomic Replicon Assay
Huh7.5.1 cells were treated with the mimic control, miR-135a or miR-122 mimic (at 25 nmol/L) for 72 h in 96-well white plates (in 5 replicates), and then transfected with JFH1-RLuc subgenomic replicon RNA (provided by C. Rice of The Rockefeller University, New York, NY) using DMRIE-C (Thermo Fisher Scientific). Cell lysates were collected after 48 h and measured for Renilla luciferase activities per manufacturer’s instructions (Promega) using a POLARstar multidetection microplate reader (BMG Labtech).
HCV IRES-Mediated Translation Assay
Huh7.5.1 cells were transfected with various miRNA mimics as described above and incubated for 72 h. Cell were then transfected with pHCV-CLX-CMV RNA (encoding HCV IRES that directs the translation of a firefly luciferase reporter gene, provided by M. Niepmann of Giessen University, Giessen, Germany). After 24 h, cell lysates were obtained and firefly luciferase activity was subsequently measured per manufacturer’s instructions (Promega) using a POLARstar multidetection microplate reader (BMG Labtech).
HCV Single-Cycle Infection Assay
The core-defective, assembly deficient single-cycle infectious HCV (HCVsc) was generated from a trans-packaging system as previously described (Masaki et al. 2010; Li et al. 2014). HCVsc is able to enter and replicate viral RNA in hepatocytes but unable to produce progeny viruses, thus recapitulating only the early stages of the HCV life cycle (entry through genome replication), but not the late stages (virion assembly and secretion). In the assay, Huh7.5.1 cells were treated with various miRNA mimics at 25 nmol/L for 72 h in 96-well white plates (in 5 replicates) before infection with HCVsc. At 48 h post-infection, cells were harvested and measured for firefly luciferase activities per manufacturer’s instructions (Promega) using a POLARstar multidetection microplate reader (BMG Labtech).
HCV P7-Luc Infection
Huh7.5.1 cells grown in 96-well white microplates (in 5 replicates) were transfected with various miRNA mimics (at 25 nmol/L) or SMARTpool siRNAs (at 50 nmol/L). After 72 h, cells were infected with HCV P7-Luc (an HCVcc system encoding a Renilla luciferase reporter, provided by C. Rice of The Rockefeller University, New York, NY). At 48 h post-infection, cell lysates were prepared and analyzed for Renilla luciferase activities per manufacturer’s instructions (Promega) using a POLARstar multidetection microplate reader (BMG Labtech).
HCV Core Staining “Part-One” (Early Steps) and “Part-Two” (Late Steps)
For “part-one” assay, Huh7.5.1 cells were transfected with miRNA mimic control (Ctrl), miR-122 mimic or miR-135a mimic at a concentration of 25 nmol/L. After 72 h, cells were infected with the HCV JFH-1 strain. At 48 h post-infection, cells were fixed and immunostained for HCV core protein expression, using a purified anti-core monoclonal antibody (produced from 6G7 hybridoma cells). Cell nuclei were stained with Hoechst 33,342 (Invitrogen) at 1:5000 in PBS. Supernatants from “part-one” wells were transferred to infected naïve Huh7.5.1 cells in a new assay plate, starting “part-two”, which underwent the same procedures as “part-one”. “Part-one” and “part-two” HCV core staining recapitulate the early (entry to viral RNA translation and replication) and the late (virion assembly and secretion) stages of HCV infection, respectively (Li et al. 2009, 2014).
Viral RNA Isolation and Quantification
Intracellular RNA was extracted from whole-cell lysates using the RNeasy Mini Kit (Qiagen). Viral RNA from supernatants (extracellular HCV RNA) was isolated using a QIAamp Viral RNA Mini Kit (Qiagen). Copy numbers of intracellular and extracellular HCV RNA were determined by Q-PCR using Verso 1-Step RT-qPCR Kit (Thermo Fisher Scientific) with previously described probe, primers, and parameters (Li et al. 2009). The relative amount of HCV RNA was normalized to the housekeeping control gene human 18S rRNA (Thermo Fisher Scientific). Q-PCR was performed on a ViiA 7 Real-Time PCR System (Applied Biosystems).
3′ UTR Assay
The LightSwitch 3′ UTR reporter GoClone constructs of various putative miR-135a targets were purchased from SwitchGear Genomics (Active Motif). Huh7.5.1 cells were transfected with miR-135a WT or MUT or with mimic control in 96-well white plates (in 5 replicates). After 24 h, cells were further transfected with 50 ng of GoClone 3′ UTR reporter plasmid using FuGENE 6 transfection reagent (Roche). Two days later, cells were lysed and total luciferase outputs were quantified using LightSwitch Luciferase Assay Reagent, according to the manufacturer’s instructions (SwitchGear Genomics) using a POLARstar multidetection microplate reader (BMG Labtech).
Gene Expression Assay
Huh7.5.1 cells were transfected with various miRNA mimics (at 25 nmol/L) or SMARTpool siRNAs (at 50 nmol/L) for 72 h and then harvested. Total cellular RNA was extracted using an RNeasy Mini Kit (Qiagen). RNA quality and quantity were assessed on a Nanodrop spectrophotometer. cDNA was synthesized from total cellular RNA using a First Strand cDNA Synthesis Kit (Roche). The mRNA levels of target genes were subsequently determined by Q-PCR using gene-specific primers and probes (IDT) and FastStart Universal Probe Master (Roche) on an ABI ViiA 7 Real-Time PCR System. Relative mRNA levels were calculated using the ΔΔCT method, with 18S rRNA (Applied Biosystems) as the internal control for normalization.
MicroRNA Quantitative Real-Time PCR (Q-PCR) Assay
Huh7.5.1 cells or cells infected with HCV were vortexed to lyse. Human liver biopsies were lysed in TissueLyser LT bead mill (Qiagen). Total miRNA was isolated using a miRNeasy Mini Kit per manufacturer’s instructions (Qiagen). RNA was reverse transcribed using the TaqMan MicroRNA Reverse Transcription Kit (Applied Biosystems) according to the manufacturer’s instructions. miR-135a expression levels were determined by Q-PCR using TaqMan Universal PCR Master Mix (Applied Biosystems) and specific miRNA primers and probes (TaqMan MicroRNA Assays, Applied Biosystems). U6 snRNA was used as an internal control.
Huh7.5.1 cells were transfected with mimic control or miR-135a mimic. Cell lysates were obtained in RIPA buffer (Sigma) with complete protease inhibitor cocktail (Roche). Lysates were incubated on ice for 30 min, and then centrifuged at 17,500 ×g for 20 min at 4 °C. Supernatants were harvested and used for western blot analysis, on a ProteinSimple’s Wes instrument, per manufacturer’s instructions. 5 µL of diluted antibody per sample was applied. Multiple antibodies used for Western blotting were obtained commercially: MYD88 rabbit polyclonal antibody (1:500) (ab2064; Abcam), RIPK2 rabbit monoclonal antibody (1:500) (Clone D10B11; Cell Signaling), β-Tubulin monoclonal antibody (1:2000) (TUB 2.1; Sigma).
Huh7.5.1 cells were seeded in 96-well white assay plates at a density of 10,000 cells per well, and then treated with various miRNA mimics or miR-122 hairpin inhibitor at a final concentration of 25 nmol/L (in 5 replicates). After 72 h, cells were harvested and lysed with 50 µL of mammalian cell lysis solution (PerkinElmer). After 5 min incubation, 50 µL of ATPlite substrate solution (PerkinElmer) were added, and the luminescence in each well was subsequently measured using a POLARstar Omega multidetection microplate reader.
Results are presented as the means ± SE (standard errors). The two-tailed unpaired Student’s t test was used for statistical analysis. The level of significance is denoted in each figure (*P < 0.05, **P < 0.01. NS, not significant).
miR-135a Transfection Enhances HCV Infection in Hepatocytes
miR-135a Preferentially Regulates HCV Genome Replication
The Mutant Form of miR-135a Lacks the miRNA’s Proviral Effect
Pan-Genotypic Effect of miR-135a on HCV Infection
In addition to modulating the replication of genotype 2a HCV as described above, we also showed that overexpression of miR-135a in Huh7.5.1 cells significantly enhanced the infection of other major HCV genotypes (including 1a, 1b, 2b, 3a and 4a) (Fig. 3D). Transfection of miR-135a MUT failed to achieve the miR-135a’s proviral effect on infection of various genotypes (Fig. 3D). Hence, miR-135a modulates HCV infection in a pan-genotypic manner that depends on the binding of its seed sequence to target mRNAs.
miR-135a Directly Targets Multiple HCV Restriction Factors in Hepatocytes
Each of these putative targets possesses a miR-135a seed sequence match site in its 3′ UTR (Fig. 4B). To evaluate whether miR-135a truly binds to these genes, we performed 3′ UTR assays. The entire 3′ UTR of the gene of interest was inserted downstream of a luciferase reporter gene (Fig. 4B). The luciferase activity was then measured after co-transfection of these 3′ UTR constructs with either the miR-135a mimic or a non-targeting control. miR-135a overexpression significantly reduced the luciferase activities of CXCL12, MYD88 and RIPK2 3′ UTR constructs, but not that of the TGFBR1 3′ UTR (Fig. 4C), suggesting regulation of the former three 3′ UTRs by miR-135a. The function of miR-135a in regulating CXCL12, MYD88 and RIPK2 3′ UTR activities was further confirmed by testing the miR-135a MUT, which did not inhibit CXCL12, MYD88 or RIPK2 3′ UTR activity, due to a loss of seed-sequence binding to these targets (Fig. 4D).
miR-135a Downregulates CXCL12, MYD88 and RIPK2 Hepatocellular Expression to Enhance HCV Replication
For RIPK2, while its 3′ UTR was subject to miR-135a regulation (Fig. 4C, 4D), its mRNA expression level did not show a decrease after miR-135a transfection (Fig. 5A). We then assessed its protein level and showed that miR-135a mimic overexpression substantially decreased RIPK2 protein level in Huh7.5.1 cells (Fig. 5C). Similarly, the protein level of an above confirmed miR-135a target–MYD88–was also considerably reduced in cells transfected with miR-135a (Fig. 5C). These data are in line with the notion that although lowered mRNA levels account for the major form of miRNA-mediated modulation, a few “outliers” in which translation is suppressed without noticeable mRNA degradation exist, such as in the case of RIPK2. We further confirmed the antiviral effects of CXCL12, MYD88 and RIPK2 that mediate miR-135a’s proviral functions. Knockdown of these host genes in Huh7.5.1 cells by siRNA treatment significantly enhanced P7-Luc HCV infection (Fig. 5D, 5E).
Collectively, the data generated a schematic map that illustrates miR-135a’s proviral function and mechanism of action. We demonstrated that miR-135a selectively targets and inhibits the expression of three critical HCV restriction factors, CXCL12, MYD88 and RIPK2, to specifically promote viral genome replication (Fig. 5F).
HCV Infection Induces miR-135a Expression in Hepatocytes and Human Liver
Cellular miRNAs have been shown to play important regulatory roles in HCV infection and virus–host interactions (Singaravelu et al. 2014). Indeed, both the HCV genome and viral host dependencies can be targeted by cellular miRNAs, which either positively or negatively regulate productive HCV infection (Sarnow et al. 2006; Singaravelu et al. 2014). HCV may also impact the expression landscape of host miRNAs and co-opt their functions to establish viral persistence and induce various pathological conditions in the liver (Schwerk et al. 2015). The exact roles and underlying mechanisms of cellular miRNAs in modulating HCV infection and associated liver diseases, nevertheless, remain to be elucidated.
Recently, applying integrative functional genomics and systems biology approaches, we globally dissected HCV–cellular miRNA interactions. We conducted a combined genome-wide miRNA mimic and hairpin inhibitor phenotypic screen, followed by miRNA–mRNA transcriptomics analyses, and identified both proviral and antiviral miRNAs that physiologically interact with HCV. These miRNAs were then functionally linked to particular steps of HCV life cycle and related viral host dependencies, thereby revealing extensive cellular miRNA–mRNA regulatory networks associated with HCV infection and propagation (Li et al. 2017). Follow-up studies towards several of the most significant and relevant miRNA hits concerning their modes of actions in modulating HCV-mediated liver pathogenesis have been carried out. These include miR-135a, a proviral cellular miRNA identified from the mimic screen (Li et al. 2017).
In the present study, we elucidated the precise function and the mode of action of miR-135a in modulating HCV infection. Through a panel of virological assays, we identified that miR-135a preferentially induces the viral genome amplification but does not alter the other stages of the HCV life cycle. We determined that miR-135a enhances the abundance of HCV RNA in hepatocytes via its antagonism of cellular antiviral pathways, by targeting and repressing the expression of several critical host restriction factors, including CXCL2, MYD88 and IRPK2, at either the transcriptional or translational level. Consistent with the role of miR-135a, these antiviral factors were shown to restrict HCV RNA replication.
Among the validated miR-135a cellular targets, CXCL2 is an antimicrobial gene serving as a ligand for the G-protein couple receptor CCR4 that plays a role in diverse cellular functions, including immune surveillance, inflammatory response, tissue homeostasis, and tumor growth and metastasis (Murphy and Heusinkveld 2018). MYD88 is a key regulator of the innate and adaptive immune responses. In particular, it serves as an essential signal transducer in the interleukin-1 and Toll-like receptor signaling pathways, which govern the activation of versatile proinflammatory genes (Akira and Takeda 2004). The serine/threonine protein kinase RIPK2 is also an important component of signaling complexes in both the innate and adaptive immune pathways. Interestingly, in response to various stimuli including viral infections, RIPK2 potently activates NF-κB and induces apoptotic cell death as a host defense mechanism (McCarthy et al. 1998). Given their versatile pathophysiological functions, these genes may mediate not only miR-135’s proviral function, but also its putative effects on multiple hepatic disorders, particularly HCV-associated liver malignancy.
One such liver malignancy is hepatocellular carcinoma (HCC), which is the predominant form of liver cancer and overall the fifth most prevalent tumor type and the second leading cause of cancer-related deaths worldwide. In the United States, HCC is the fastest growing cause of cancer-related mortality. Chronic infection with HCV is the leading risk factor for HCC. HCV is unique among tumor viruses in that it does not encode oncoproteins nor there is integration of the viral genome into host chromosomal DNA. Thus, the virus initiates and promotes HCC development mainly through indirect effects by deregulating various cellular processes, including hepatocyte proliferation, steatosis, oxidative stress, inflammation, immune responses, and fibrosis/cirrhosis. The specific molecular mechanisms supporting HCV-mediated HCC initiation and progression, nevertheless, are poorly understood. To further explore these mechanisms we looked into miR-135a, the expression of which has been shown broadly induced across various cancer types, including HCC (Van Renne et al. 2018). Elevated miR-135a expression, on the other hand, promotes cancer cell invasion and thus contribute to metastasis and progression of HCC (Liu et al. 2012). The mechanisms by which miR-135a exerts these oncogenic effects remain unclear.
Accumulating evidence has demonstrated that miRNA machinery constitutes an important part of viral host dependencies, and host miRNAs may modulate viral infections either directly, through targeting viral genomes, or indirectly, through regulation of virus-associated cellular pathways (Sarnow et al. 2006). Viruses, such as Epstein–Barr virus (EBV), enterovirus, and eastern equine encephalitis virus, can benefit from the expression of certain miRNAs, and may further induce these miRNAs for productive infection and pathogenesis (Trobaugh et al. 2014). Endogenous cellular miRNAs can also exert significant antiviral effects on several human viruses, including vesicular stomatitis virus (VSV) and the retrovirus human immunodeficiency virus (HIV) (Lecellier et al. 2005). Nevertheless, many viruses seem to be resistant to suppression or augmentation by the host miRNome during their replication cycle (Bogerd et al. 2014). These viruses may have evolved strategies to impede endogenous miRNA functions or to avoid direct miRNA targeting (Cullen 2013). On the other hand, given the versatile regulatory roles of miRNAs in any given biological process (such as viral infection), multiple concurrent proviral or antiviral signals may co-exist and be tightly controlled by the miRNA machinery to maintain a balanced state.
HCV infection closely interacts with the miRNA machinery. Besides miR-122, and miR-135a characterized in this study, a number of other cellular miRNAs have been shown to be critical regulators of HCV infection. Among them, miR-196 may target Bach1, and hence up-regulate the antiviral factor HMOX1 to restrict HCV replication (Hou et al. 2010). miR-29, a cellular miRNA implicated in development of liver fibrosis (Roderburg et al. 2011), was demonstrated to block HCV propagation and its expression is down-regulated by HCV (Bandyopadhyay et al. 2011). In addition, miR-199a-3p, an HCC-suppressive miRNA (Hou et al. 2011), possesses a conserved binding site in the HCV 5′ UTR. And overexpression of this miRNA significantly inhibits HCV infection (Murakami et al. 2009). Besides miR-199a, several other miRNAs including miR-448 and miR-196, with putative target sequences on the viral genome, have been shown to attenuate HCV genome replication (Pedersen et al. 2007). However, direct functional interactions of these miRNAs with the viral genome have not been shown. Interestingly, some of these miRNAs seem to be induced by interferons, with inhibition of their activities impairing IFN’s antiviral effect against HCV (Pedersen et al. 2007). Hence, HCV may manipulate these IFN-activated miRNAs to limit replication to a level appropriate for persistent viral infection.
In summary, as major factors in controlling various cellular processes, miRNAs represent a compelling subject of investigation in the study of HCV infection and virus-mediated liver diseases. Understanding miRNA-associated mechanisms and major signaling pathways involved in HCC pathogenesis is instrumental for target-directed drug discovery to optimize the current treatment regimen for liver cancer.
We thank Dr. M.G. Ghany for providing human liver biopsy samples. We also thank Drs. C.M. Rice, F.V. Chisari, T. Wakita, F.-L. Cosset, M. Niepmann and T. Suzuki for their generosity in providing various reagents. This work was supported by the Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases, US National Institutes of Health.
QL and TJL conceived and designed the study. CS, BL, LH and QL conducted experiments. CS, BL, LH, TJL and QL analyzed data. QL and TJL wrote the paper with the input from all other co-authors.
Compliance with Ethics Standards
Conflict of interest
The authors declare that they have no conflict of interest.
Animal and Human Rights Statement
All patients that provided liver biopsies for this study submitted written informed consent and the protocol was approved by the Institutional Review Board of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). The procedures followed were in accordance with the ethical standards of the institutional committees on human experimentation.
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