MicroRNAs (miRNAs) are small non-coding RNAs 18-25 nucleotides in length, transcribed from non-protein- coding genes or introns, which regulate gene expression through repressing translation and cleaving their target mRNAs by binding to complementary sites in their 3'-untrans-

Accumulating evidence indicates that microRNAs are involved in multiple processes in cancer development and progression. microRNA-26a (miR-26a) has been identified as a tumor suppressor and its downregulation is associated with poor prognosis in several types of human cancer. However, the specific function of miR-26a in osteosarcoma remains unclear. In the present study, we found that the expression of miR-26a in osteosarcoma tissues and cell lines was much lower than that in the normal controls, respectively. In addition, downregulation of miR-26a more frequently occurred in osteosarcoma specimens with adverse clinical stage and with the presence of distant metastasis. Moreover, multivariate survival analyses demonstrated that loss of miR-26a is an independent prognostic factor for both disease-free and overall survival in osteosarcoma. In addition, restoration of miR-26a expression inhibited the invasion and migration in osteosarcoma cells, and miR-26a directly inhibited enhancer of zeste homolog 2 (EZH2) expression by targeting its 3'-UTR. Moreover, EZH2 was upregulated and inversely correlated with miR-26a expression in the osteosarcoma tissues. Thus, for the first time, we provide convincing evidence that downregulation of miR-26a is associated with tumor aggressiveness and tumor metastasis, and miR-26a inhibits cell migration and invasion by targeting the EZH2 gene in osteosarcoma. Thus, miR-26a is an independent prognostic marker for osteosarcoma patients. Introduction Osteosarcoma, mainly arising from the metaphysis of the long bones, is the most common primary malignancy of bone in adolescents and young adults, with an estimated worldwide yearly incidence rate of 4 million (1,2). Despite current therapeutic strategies combining adjuvant chemotherapy, surgery and sometimes radiotherapy, the prognosis of osteosarcoma patients remains poor, since ~80% of patients eventually develop recurrent metastatic osteosarcoma following surgical treatment (3), and the 5-year survival rate of these patients is only 50-60% (4). Although recent developments in molecular biology have provided insight into the molecular mechanisms of osteosarcoma, the fundamental molecular mechanisms underlying metastasis in osteosarcoma have not been fully elucidated. Therefore, it is essential to identify metastasisassociated molecules as effective drug targets and to enhance the understanding of the mechanisms underlying the metastasis of osteosarcoma. MicroRNAs (miRNAs) are small non-coding RNAs 18-25 nucleotides in length, transcribed from non-proteincoding genes or introns, which regulate gene expression through repressing translation and cleaving their target mRNAs by binding to complementary sites in their 3'-untranslated region (3'-UTR). It has been demonstrated that aberrant expression of miRNAs cause them to function as tumor suppressors or oncogenes according to the roles of their target genes (5,6). Particularly, miRNAs can regulate various cellular processes of tumor cells, including differentiation, progression, apoptosis, proliferation, migration and invasion (7). To date, several human miRNAs such as miR-335, miR-145 and miR-128 have been shown to be dysregulated in osteosarcoma (7-9), and contribute to the development and progression of osteosarcoma. Emerging data reveal that microRNA-26a (miR-26a) is downregulated and may serve as a potential tumor suppressor in several distinct cancer types including nasopharyngeal carcinoma, breast cancer, thyroid anaplastic carcinomas and hepatocellular carcinoma (5,10-14). Importantly, miR-26a was found to be highly expressed in lymph node metastatic tumors as compared with primary tumors and enhanced lung cancer cell migration and invasion (15). miR-26a can suppress cell differentiation, migration and invasion by targeting a number Downregulation of microRNA-26a is associated with metastatic potential and the poor prognosis of osteosarcoma patients QI-CHUN SONG1, ZHI-BIN SHI1, YONG-TAO ZHANG1, LE JI1, KUN-ZHENG WANG1, DA-PENG DUAN2 and XIAO-QIAN DANG1 1Department of Orthopedics, The Second Affiliated Hospital, Xi'an Jiaotong University, Xi'an, Shaanxi 710004; 2Department of Orthopedics, Shaanxi Provincial People's Hospital, Xi'an, Shaanxi 710068, P.R. China Received November 16, 2013; Accepted December 30, 2013 DOI: 10.3892/or.2014.2989 Correspondence to: Professor Xiao-Qian Dang, Department of Orthopedics, The Second Affiliated Hospital, Xi'an Jiaotong University, 157 Xiwu Road, Xi'an, Shaanxi 710004, P.R. China E-mail: [email protected] Abbreviations: miR-26a, miRNA-26a; EZH2, enhancer of zeste homolog 2; PcG, polycomb group; WT, wild-type; MT, mutant; 3'-UTR, 3'-untranslated region; qRT-PCR, quantitative real-time polymerase chain reaction