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Review ArticleMicroRNAs as Important Players and Biomarkers inOral Carcinogenesis
Anjie Min,1 Chao Zhu,1,2 Shuping Peng,3 Saroj Rajthala,4,5
Daniela Elena Costea,4,5,6 and Dipak Sapkota4,5,7
1Department of Oral and Maxillofacial Surgery, Xiangya Hospital, Central South University, Changsha, China2School of Stomatology, Central South University, Changsha, China3Cancer Research Institute, Central South University, Changsha, China4The Gade Laboratory for Pathology, Department of Clinical Medicine, Faculty of Medicine and Dentistry, University of Bergen,5021 Bergen, Norway5Centre for Cancer Biomarkers (CCBIO), Faculty of Medicine and Dentistry, University of Bergen, 5021 Bergen, Norway6Department of Pathology, Haukeland University Hospital, 5021 Bergen, Norway7Department of Oncology and Medical Physics, Haukeland University Hospital, 5021 Bergen, Norway
Correspondence should be addressed to Dipak Sapkota; email@example.com
Received 6 February 2015; Revised 14 May 2015; Accepted 18 May 2015
Academic Editor: David Pauza
Copyright 2015 Anjie Min et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Oral cancer, represented mainly by oral squamous cell carcinoma (OSCC), is the eighth most common type of humancancer worldwide. The number of new OSCC cases is increasing worldwide, especially in the low-income countries, and theprognosis remains poor in spite of recent advances in the diagnostic and therapeutic modalities. MicroRNAs (miRNAs), 1825nucleotides long noncoding RNA molecules, have recently gained significant attention as potential regulators and biomarkers forcarcinogenesis. Recent data show that several miRNAs are deregulated in OSCC, and they have either a tumor suppressive or anoncogenic role in oral carcinogenesis. This review summarizes current knowledge on the role of miRNAs as tumor promotors ortumor suppressors in OSCC development and discusses their potential value as diagnostic and prognostic markers in OSCC.
Head and neck squamous cell carcinoma (HNSCC) consistsof a heterogeneous group of malignancies arising from oralcavity, nasal cavity, paranasal sinuses, pharynx, larynx, andsalivary glands. Oral cancer, represented mainly by oralsquamous cell carcinoma (OSCC), is the most commontype of HNSCC. OSCC is the eighth most common cancerworldwide accounting for more than 300,000 new cases and145,000 deaths in 2012 . Usually, OSCC detection dependson the clinical examination of oral cavity, followed by a biopsyfor histological analysis. However, despite the easy accessfor visual examination, OSCC is often detected at advancedstages leading to severely reduced patient survival. In spite ofthe recent advances in diagnosis and treatment modalities,less than 50% of OSCC patients survive for 5 years . Late
diagnosis, regional lymph node metastasis, and recurrencesare the major causes related to the poor prognosis andreduced survival for OSCC patients [3, 4]. Thus, reliablemolecular markers that can (i) provide earlier and moreprecise OSCC diagnosis, (ii) predict prognosis, and (ii) assignpatients to the best-targeted treatment available are urgentlyneeded.
For almost three to four decades, changes in proteincoding tumor suppressor genes and/or oncogenes have beenthought to be the main drivers of tumor development [5,6]. However, the recent discovery of thousands of genesthat transcribe noncoding RNAs (including miRNAs) makesit obvious that cancer biology is even more complex thaninitially expected. Several layers of molecular regulators(e.g., mRNA, miRNA, and protein) are involved in thedevelopment and maintenance of cancerous phenotypes.
Hindawi Publishing CorporationBioMed Research InternationalVolume 2015, Article ID 186904, 10 pageshttp://dx.doi.org/10.1155/2015/186904
2 BioMed Research International
Regulation of geneexpression
Activation of oncogenesand/or suppression of
tumor suppressor genes
Proliferation and survival,
metastatic potential of oralkeratinocytes
apoptosis and invasion and/or
Figure 1: Schematic illustration demonstrating biogenesis and function of miRNA. miRNA genes are transcribed into primary miRNA (pri-miRNA) by RNA polymerase III.These miRNAs are further converted into second precursors (pre-miRNA) by Drosha and are exported intocytoplasm by Exportin 5. Additional processing by Dicer produces miRNA:miRNA duplex. Only one strand of miRNA:miRNA duplex ispreferentially assembled into the RNA-induced silencing complex (RISC). RISC acts on target mRNA(s) and leads to either translationalrepression or mRNA cleavage. Suppression of tumor suppressive genes and/or activation of oncogenes by miRNA lead to excessive cellproliferation and survival, increased antiapoptosis, and enhanced invasive and metastatic potential of oral keratinocytes, resulting intoinvasive cancerous growth.
Among them, miRNAs, 1825 nucleotides long, noncod-ing RNA molecules , have recently gained significantattention as potential regulators and biomarkers for humancarcinogenesis. At the molecular level, miRNA binds to3-untranslated region (3-UTR) of target mRNA(s) andsuppresses its expression by either translational repression ormRNA cleavage  (Figure 1). A single miRNA can regulateexpression and/or function of hundreds of targetmRNAs andproteins and regulates several biological processes (e.g., cellproliferation, differentiation,migration, apoptosis, and signaltransduction) important for cancer development [8, 1113](Figure 1).
Many recent studies have shown deregulated expressionof miRNAs in OSCC and OSCC-derived cell-lines comparedto their normal counterparts, indicating their potential role inoral cancer development. Accordingly, several miRNAs havebeen shown to function either as tumor suppressors or astumor promoters in OSCCs (reviewed in [14, 15]). In additionto their key biological functions in OSCC tumorigenesis,expression levels of several of miRNAs have been shown tocorrelate with clinicopathological variables  and to have adiagnostic and prognostic value in OSCC [15, 17]. For thesereasons, miRNA has been a hot topic in cancer research forthe last few years and several studies about miRNAs in OSCC
have been published recently, as summarized in Table 1. Thecurrent review aims to highlight the oncogenic and tumorsuppressive roles of miRNAs in OSCC development anddiscusses their potential value as diagnostic and prognosticmarkers for OSCC management.
Literature search was performed by using the PubMeddatabase. Following key words were used for the litera-ture search: oral cancer and miRNA, oral cancer andmicroRNA, oral squamous cell carcinoma andmiRNA, andoral squamous cell carcinoma and microRNA. Exclusioncriteria were articles not related to OSCC/HNSCC and/ormiRNA, purely descriptive articles, articles lacking clinicalpathological correlation, and/or articles for which full textswere not available in English. Only clinically relevant articlespublished within April 2015 were included in this review.Additionally, individual articles retrieved manually from thereference list of the relevant papers were also included.
3. miRNAs as Oncogenes in OSCC
A number of miRNAs have been shown to be upregulatedin OSCC and to function as oncogenes. A well-studied
BioMed Research International 3
Table 1: Summary of miRNAs and associated signal pathways/target genes in OSCC/HNSCC.
miRNA Up/downregulation Target genes/associated pathways Ref.
PDCD4 TPMI RECK CLU DKK2-Wnt/-catenin Smad7-TGF1 HA/CD44-Nanog/Stat3-PDCD4, IAPs 
miR-31 Up FIH-HIF-EVGF 
miR-31 Up FGF3 RhoA 
miR-134 Up WWOX miR-146a Up IRK1, TRAF6, and NUMB miR-155 Up CDC73 
miR-7 Up RECK IGF1R-Akt 
miR-9 Down CXCR4-Wnt/-catenin miR-17/20a Down ITG8 miR-29a Down MMP2 
miR-34 Down E2F3, survivin, and VEGF SIRT6 
miR-99a Down IGF1R miR-124 Down ITGB1 miR-125b Down ICAM2 
miR-138 DownFOSL1 VIM, ZEB2, EZH2 RhoC, and RoCK2 
miR-140-5p Down ADAM10, ERBB4, PAX6, and LAMC1 miR-145 Down c-Myc, Cdk6 
miR-181a Down K-ras Twist1 
miR-205 Down IL-24, caspase-3/-7, and Axin-2 
miR-218 Down mTOR-Rictor-Akt miR-320 Down HIF-1-NRP1-VEGF 
miR-357 Down CIP2A-MYC AEG-1/MTDH 
miR-419-5p Down GIT1 EGFR-ERK1/2-MMP2/9
miR-483-3p API5, BRIC5, and RAN miR-196a Up MAMDC2 miR-26a/b Down TMEM184B 
miRNA, the miR-21, has been shown to be overexpressedand to regulate several biological functions in OSCC [16,1820]. Overexpression of miR-21 has also been observedin oral premalignant lesions (oral leukoplakia) comparedto normal oral mucosa, indicating that alteration in miR-21 could be an earlier event in OSCC progression . Anumber of in vitro and in vivo experimental data have demon-strated an oncogenic role of miR-21 in OSCC by promoting
cell proliferation , invasion [18, 23], antiapoptosis ,and chemoresistance . These oncogenic functions wereshown to be regulated by miR-21-mediated downregulationof several established tumor suppressor molecules, includ-ing PTEN , programmed cell death 4 (PDCD4) ,tropomyosin , reversion-inducing cysteine-rich proteinwith kazal motifs (RECK) , and dickkopf 2 (DKK2) .In addition to the functional role