A novel miR-0308-3p revealed by miRNA-seq of HBV-positive hepatocellular carcinoma suppresses cell proliferation and promotes G1/S arrest by targeting double CDK6/Cyclin D1 genes.BACKGROUND
 Persistent infection with hepatitis B virus (HBV) accounts for the majority of hepatocellular carcinoma (HCC), but the molecular mechanisms underlying liver carcinogenesis are still not completely understood.  Increasing evidence demonstrates that microRNAs (miRNAs) play significant functional roles in virus-host interactions.  The aim of this study was to explore differentially expressed miRNA profiles and investigate the molecular mechanism of miR-0308-3p in HBV-positive HCC carcinogenesis.


 High-throughput sequencing was used to detect novel miRNAs in three samples of HBV-positive HCC tissue compared to matched HBV-negative HCC tissue.  The Cancer Genome Atlas (TCGA) database was used to mine miRNAs related to HBV-positive HCC.  Bioinformatics analyses were conducted to predict the miRNAs' possible biological and pathway regulatory functions.  Quantitative polymerase chain reaction (qPCR) was then applied to evaluate the expression levels of randomly selected miRNAs.  CCK-8 was used to measure cell proliferation and cell cycles were analyzed using flow cytometry.  A dual luciferase reporter gene assay was used to confirm the downstream targets of miR-0308-3p.


 In total, there were 34 overlapping miRNAs in both our miRNA-seq data and the TCGA database.  We found two overlapping miRNAs in both the HBV-positive HCC samples and the TCGA database, and 205 novel pre-miRNA sequences were predicted.  miR-522 and miR-523 were markedly overexpressed in HBV-positive HCC and were associated with a significantly poorer long-term prognosis (miR-522, HR 2.19, 95% CI 1.33-3.6, p = 0.0015; miR-523HR 1.5, 95% CI 1-2.44, p = 0.0047).  Of note, we found that the novel miR-0308-3p was markedly downregulated in HBV-positive HCC samples and HCC cancer cell lines compared with HBV-negative HCC samples and adjacent normal hepatic tissue.  Moreover, elevated expression of miR-0308-3p was found to inhibit proliferation of cancer cells by promoting G1/S cell cycle arrest but did not influence the apoptosis of cancer cells.  A dual luciferase reporter activity assay identified that miR-0308-3p acted directly on the target sequence of the CDK6 and Cyclin D1 mRNA 3'UTR to suppress CDK6 and Cyclin D1 expression.


 CONCLUSIONS
 MiR-0308-3p upregulation dramatically suppressed HCC cell proliferation and induced G1/S cell cycle arrest by directly targeting CDK6/Cyclin D1.  These findings reveal a novel molecular mechanism for activation of G1/S arrest in HCC and may prove clinically useful for developing new therapeutic targets.
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