MicroRNA silencing improves the tumor specificity of adenoviral transgene expression. Cancer Gene Ther. 2012;19:451-459. [PMID: 22555510 PMCID: PMC3380162 DOI: 10.1038/cgt.2012.16]

The role and mechanism of action of microRNA-122 in cancer: Focusing on the liver

microRNA-122 (miR-122) is a highly conserved microRNA that is predominantly expressed in the liver and plays a critical role in the regulation of liver metabolism. Recent studies have shown that miR-122 is involved in the pathogenesis of various types of cancer, particularly liver cancer. In this sense, The current findings highlighted the potential role of miR-122 in regulating many vital processes in cancer pathophysiology, including apoptosis, signaling pathway, cell metabolism, immune system response, migration, and invasion. These results imply that miR-122, which has been extensively studied for its biological functions and potential therapeutic applications, acts as a tumor suppressor or oncogene in cancer development. We first provide an overview and summary of the physiological function and mode of action of miR-122 in liver cancer. We will examine the various signaling pathways and molecular mechanisms through which miR-122 exerts its effects on cancer cells, including the regulation of oncogenic and tumor suppressor genes, the modulation of cell proliferation and apoptosis, and the regulation of metastasis. Most importantly, we will also discuss the potential diagnostic and therapeutic applications of miR-122 in cancer, including the development of miRNA-based biomarkers for cancer diagnosis and prognosis, and the potential use of miR-122 as a therapeutic target for cancer treatment.

The application of oncolytic viruses in cancer therapy

Oncolytic therapy is a treatment method used to directly combat tumor cells by modifying the genes of naturally occurring low pathogenic viruses to form "rhizobia" virus. By taking the advantage of abnormal signal pathways in cancer cells, it selectively replicates in tumor cells leading to tumor cell lysis and death. At present, clinical studies widely employ biomolecular technology to transform oncolytic viruses to exert stronger oncolytic effects and reduce their adverse reactions. This review summarizes the current progresses and the molecular mechanism of oncolytic viruses towards tumor treatment and management.

Effect of Transgene Location, Transcriptional Control Elements and Transgene Features in Armed Oncolytic Adenoviruses

Clinical results with oncolytic adenoviruses (OAds) used as antitumor monotherapies show limited efficacy. To increase OAd potency, transgenes have been inserted into their genome, a strategy known as "arming OAds". Here, we review different parameters that affect the outcome of armed OAds. Recombinant adenovirus used in gene therapy and vaccination have been the basis for the design of armed OAds. Hence, early region 1 (E1) and early region 3 (E3) have been the most commonly used transgene insertion sites, along with partially or complete E3 deletions. Besides transgene location and orientation, transcriptional control elements, transgene function, either virocentric or immunocentric, and even the codons encoding it, greatly impact on transgene levels and virus fitness.

Synergistic and independent action of endogenous microRNAs 122a and 199a for post-transcriptional liver detargeting of gene vectors

In hepatocellular carcinoma (HCC), which usually develops in a cirrhotic liver, treatments preserving normal liver function and viability are vitally important. Here, we utilise the differential expression of miRNAs 122a and 199a between normal hepatocytes and HCC to generate vectors harbouring their binding sites for hepatocyte detargeting. Using a reporter gene, we observed a synergistic detargeting of cells expressing both miRNAs as well as cells expressing either of the miRNAs; while expression was retained in HCC cells negative for both miRNA122a and miRNA199a. Mimics and inhibitors for individual miRNAs were used to confirm these results. Furthermore, suicide gene therapy with cytosine deaminase (CD)/5-fluorocytosine system resulted in limited killing of cells expressing either of the two miRNAs. Finally, we report feasibility of using adeno associated virus (AAV) based vectors for delivery of this dual regulated gene delivery system. These results present a novel dual targeted system whereby miRNA122a and miRNA199a act either synergistically or independently in regulating transgene expression with vectors harbouring binding sites of both miRNAs and have implications in detargeting vectors from multiple cell types in the liver.

miRNA122a regulation of gene therapy vectors targeting hepatocellular cancer stem cells

In this study, we report a miRNA122a based targeted gene therapy for hepatocellular cancer stem cells (CSCs). First, we assessed the levels of miRNA122a in normal human hepatocytes, a panel of hepatocellular carcinoma (HCC) cell lines and hepatocellular CSCs observing its significant downregulation in HCC and CSCs. The miRNA122a binding site was then incorporated at the 3'-UTR of reporter genes gaussia luciferase (GLuc) and eGFP which resulted in significant hepatocyte detargeting. Using this strategy for the delivery of gene directed enzyme prodrug therapy (GDEPT) utilizing the cytosine deaminase/5-fluorocytosine (CD/5-FC) system, we showed significant killing in cells with low or no miRNA122a while those cells, such as hepatocytes with high miRNA122a were largely spared. Next, we showed that CSC enriched tumorspheres exhibit a significant downregulation of miRNA122a expression providing a rational to exploit its binding site for targeted gene delivery. Using plasmids harboring reporters GLuc and eGFP with or without miR122a binding sites, we showed high reporter expression in the CSCs and little reported expression in the non-enriched cultures. Finally, we demonstrate the efficacy of miRNA122a based post-transcriptionally targeted GDEPT for hepatocellular CSCs.

MicroRNA-regulated viral vectors for gene therapy

Safe and effective gene therapy approaches require targeted tissue-specific transfer of a therapeutic transgene. Besides traditional approaches, such as transcriptional and transductional targeting, microRNA-dependent post-transcriptional suppression of transgene expression has been emerging as powerful new technology to increase the specificity of vector-mediated transgene expression. MicroRNAs are small non-coding RNAs and often expressed in a tissue-, lineage-, activation- or differentiation-specific pattern. They typically regulate gene expression by binding to imperfectly complementary sequences in the 3' untranslated region (UTR) of the mRNA. To control exogenous transgene expression, tandem repeats of artificial microRNA target sites are usually incorporated into the 3' UTR of the transgene expression cassette, leading to subsequent degradation of transgene mRNA in cells expressing the corresponding microRNA. This targeting strategy, first shown for lentiviral vectors in antigen presenting cells, has now been used for tissue-specific expression of vector-encoded therapeutic transgenes, to reduce immune response against the transgene, to control virus tropism for oncolytic virotherapy, to increase safety of live attenuated virus vaccines and to identify and select cell subsets for pluripotent stem cell therapies, respectively. This review provides an introduction into the technical mechanism underlying microRNA-regulation, highlights new developments in this field and gives an overview of applications of microRNA-regulated viral vectors for cardiac, suicide gene cancer and hematopoietic stem cell therapy, as well as for treatment of neurological and eye diseases.

Evaluation of miR-122-regulated suicide gene therapy for hepatocellular carcinoma in an orthotopic mouse model

OBJECTIVE

Intratumoral administration of adenoviral vector encoding herpes simplex virus (HSV) thymidine kinase (TK) gene (Ad-TK) followed by systemic ganciclovir (GCV) is an effective approach in treating experimental hepatocellular carcinoma (HCC). However, hepatotoxicity due to unwanted vector spread and suicide gene expression limited the application of this therapy. miR-122 is an abundant, liver-specific microRNA whose expression is decreased in human primary HCC and HCC-derived cell lines. These different expression profiles provide an opportunity to induce tumor-specific gene expression by miR-122 regulation.

METHODS

By inserting miR-122 target sequences (miR-122T) in the 3' untranslated region (UTR) of TK gene, we constructed adenovirus (Ad) vectors expressing miR-122-regulated TK (Ad-TK-122T) and report genes. After intratumoral administration of Ad vectors into an orthotopic miR-122-deficient HCC mouse model, we observed the miR-122-regulated transgene expression and assessed the antitumor activity and safety of Ad-TK-122T.

RESULTS

Insertion of miR-122T specifically down-regulated transgene expression in vitro and selectively protected the miR-122-positive cells from killing by TK/GCV treatment. Insertion of miR-122T led to significant reduction of tansgene expression in the liver without inhibition of its expression in tumors in vivo, resulting in an 11-fold improvement of tumor-specific transgene expression. Intratumoral injection of Ad vectors mediated TK/GCV system led to a vector dosage-dependent regression of tumor. The insertion of miR-122T does not influence the antitumor effects of suicide gene therapy. Whereas mice administrated with Ad-TK showed severe lethal hepatotoxicity at the effective therapeutic dose, no liver damage was found in Ad-TK-122T group.

CONCLUSIONS

miR-122-regulated TK expression achieved effective anti-tumor effects and increased the safety of intratumoral delivery of adenovirus-mediated TK/GCV gene therapy for miR-122-deficient HCC.

Targeted expression of suicide gene by tissue-specific promoter and microRNA regulation for cancer gene therapy

In order to realise the full potential of cancer suicide gene therapy that allows the precise expression of suicide gene in cancer cells, we used a tissue specific Epithelial cell adhesion molecule (EpCAM) promoter (EGP-2) that directs transgene Herpes simplex virus–thymidine kinase (HSV-TK) expression preferentially in EpCAM over expressing cancer cells. EpCAM levels are considerably higher in retinoblastoma (RB), a childhood eye cancer with limited expression in normal cells. Use of miRNA regulation, adjacent to the use of the tissue-specific promoter, would provide the second layer of control to the transgene expression only in the tumor cells while sparing the normal cells. To test this hypothesis we cloned let-7b miRNA targets in the 3’UTR region of HSV-TK suicide gene driven by EpCAM promoter because let-7 family miRNAs, including let-7b, were found to be down regulated in the RB tumors and cell lines. We used EpCAM over expressing and let-7 down regulated RB cell lines Y79, WERI-Rb1 (EpCAM ^+ve/let-7b^{down-regulated}), EpCAM down regulated, let-7 over expressing normal retinal Müller glial cell line MIO-M1(EpCAM ^−ve/let-7b^up-regulated), and EpCAM up regulated, let-7b up-regulated normal thyroid cell line N-Thy-Ori-3.1(EpCAM ^+ve/let-7b^up-regulated) in the study. The cell proliferation was measured by MTT assay, apoptosis was measured by probing cleaved Caspase3, EpCAM and TK expression were quantified by Western blot. Our results showed that the EGP2-promoter HSV-TK (EGP2-TK) construct with 2 or 4 copies of let-7b miRNA targets expressed TK gene only in Y79, WERI-Rb-1, while the TK gene did not express in MIO-M1. In summary, we have developed a tissue-specific, miRNA-regulated dual control vector, which selectively expresses the suicide gene in EpCAM over expressing cells.

MicroRNA-regulated non-viral vectors with improved tumor specificity in an orthotopic rat model of hepatocellular carcinoma

In hepatocellular carcinoma (HCC), tumor specificity of gene therapy is of utmost importance to preserve liver function. MicroRNAs (miRNAs) are powerful negative regulators of gene expression and many are downregulated in human HCC. We identified seven miRNAs that are also downregulated in tumors in a rat hepatoma model (P<0.05) and attempted to improve tumor specificity by constructing a panel of luciferase-expressing vectors containing binding sites for these miRNAs. Attenuation of luciferase expression by the corresponding miRNAs was confirmed across various cell lines and in mouse liver. We then tested our vectors in tumor-bearing rats and identified two miRNAs, miR-26a and miR-122, that significantly decreased expression in liver compared with the control vector (6.40 and 0.26%, respectively; P<0.05). In tumor, miR-122 had a nonsignificant trend towards decreased (∼50%) expression, whereas miR-26 had no significant effect on tumor expression. To our knowledge, this is the first work using differentially expressed miRNAs to de-target transgene expression in an orthotopic hepatoma model and to identify miR-26a, in addition to miR-122, for de-targeting liver. Considering the heterogeneity of miRNA expression in human HCC, this information will be important in guiding development of more personalized vectors for the treatment of this devastating disease.

Molecular imaging strategies for in vivo tracking of microRNAs: a comprehensive review

MicroRNAs (miRNAs) are single-stranded non-coding RNAs of ~22 nucleotides, which can negatively regulate gene expression through induction of mRNA degradation and/or post-transcriptional gene silencing. MiRNAs are key factors in the regulation of many biological processes such as cell proliferation, differentiation, and death. Since miRNAs are known to be in close association with cancer development, non-invasive imaging of miRNA expression and/or activity is of critical importance, for which conventional molecular biology techniques are not suitable or applicable. Over the last several years, various molecular imaging techniques have been investigated for imaging of miRNAs. In this review article, we summarize the current state-of-the-art imaging of miRNAs, which are typically based on fluorescent proteins, bioluminescent enzymes, molecular beacons, and/or various nanoparticles. Non-invasive imaging of miRNA expression and/or biological activity is still at its infancy. Future research on more clinically relevant, non-toxic techniques is required to move the field of miRNA imaging into clinical applications. Non-invasive imaging of miRNA is an invaluable method that can not only significantly advance our understandings of a wide range of human diseases, but also lead to new and more effective treatment strategies for these diseases.