Downregulation of microRNA 99 a in oral squamous cell carcinomas contributes to the growth and survival of oral cancer cells

microRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression by either translational inhibition or mRNA degradation. miRNAs play pivotal roles in physiological functioning and pathological progression. The function of microRNA-99a (miR-99a) in oral squamous cell carcinoma (OSCC) remains unclear. In the present study, we investigated the roles of miR-99a in OSCC development and the underlying mechanisms in 25 cases of primary OSCC tissues and Tca-8113 cells. The cells were analyzed using FACS analysis and western blotting. Results showed that the expression levels of miR-99a were markedly decreased in OSCC tissues compared with the adjacent non-tumor tissues (n=25). The results of in vitro experiments showed that miR-99a mimics significantly inhibited the proliferation of Tca-8113 cells, a tongue squamous carcinoma cell line, and that miR-99a mimics markedly induced the apoptosis of Tca-8113 cells. Furthermore, we demonstrated that mammalian target of rapamycin (mTOR) was directly targeted by miR-99a, as the overexpression of miR-99a in Tca-8113 cells downregulated the protein expression level of mTOR. Thus, our findings suggest that the downregulation of miR-99a in OSCC tissues is associated with tumor development. miR-99a regulates the growth and survival of OSCC cells and may be exploited as a biomarker and therapeutic target for patients with OSCC. Introduction MicroRNAs (miRNAs) are short regulatory RNA molecules with an average length of 22 nucleotides. When miRNAs are integrated into RNA-induced silencing complexes, the molecules bind to a partially complementary sequence in the 3'-untranslated regions (3'-UTRs) of target mRNAs and regulate gene expression by inhibiting translation and destabilizing transcripts (1). Beyond the involvement in diverse biological processes, including cell growth, apoptosis (2), development (3), differentiation (4) and endocrine homeostasis (5), it has been well demonstrated that the deregulation or dysfunction of miRNAs contributes to cancer development. In recent years, large high-throughput studies in patients revealed that miRNA profiling has the potential to classify tumors with high accuracy. In addition, miRNA alterations have been reported to be associated with specific clinical phenotypes, including disease progression or recurrence, development of metastases and post-operative outcome. The results of functional studies, some of which involved animal models, indicate that miRNAs act as tumor suppressors and oncogenes (6). It has been proposed that alterations in miRNA genes play a critical role in the pathophysiology of various types of human cancer (7). Oral squamous cell carcinomas (OSCCs) account for over 40% of head and neck malignancies and rank sixth worldwide in incidence. OSCCs have a particularly poor prognosis due to their invasive nature. Despite advances in the fields of oncology and surgery, the five-year survival rates of OSCC are less than 50% and have remained unchanged in the last three decades (8). New molecular insights into the mechanisms of cancer formation and progression are likely to aid the improvement of diagnosis and prognosis and the development of new therapies for treating oral cancer. Using microarrays or quantitative reverse transcriptionpolymerase chain reaction (qRT-PCR), a number of studies have analyzed miRNA expression profiles in OSCC and head and neck squamous cell carcinoma (HNSCC) (9). Certain functional studies have revealed miRNA-regulated pathways in OSCC proliferation or apoptosis, metastasis and Downregulation of microRNA 99a in oral squamous cell carcinomas contributes to the growth and survival of oral cancer cells BIN YAN1*, QIHONG FU2*, LIHUA LAI3*, XINRAN TAO3, YANGHONGHONG FEI3, JIANGEN SHEN3, ZUODONG CHEN1 and QINGQING WANG3 1Department of Oral and Maxillofacial Surgery, Ningbo First Hospital, Ningbo 315000; 2Department of Stomatology, Hangzhou Normal University Clinical Medicine College, Hangzhou 310036; 3Institute of Immunology, Zhejiang University School of Medicine, Hangzhou 310058, P.R. China Received February 21, 2012; Accepted June 20, 2012 DOI: 10.3892/mmr.2012. 971 Correspondence to: Professor Qingqing Wang, Institute of Immunology, Zhejiang University School of Medicine, 866 Yu Hang Tang Road, Hangzhou 310058, P.R. China E-mail: wqq@zju.edu. cn *Contributed equally