Long non-coding RNA MEG 3 induces cell apoptosis in esophageal cancer through endoplasmic reticulum stress

Long non-coding RNAs (lncRNAs) play important roles in diverse biological processes, such as cell growth, apoptosis and migration. Although downregulation of lncRNA MEG3 has been identified in several cancers, little is known about its role in esophageal squamous cell carcinoma (ESCC). The aim of the present study was to detect MEG3 expression in clinical ESCC tissues, investigate its biological functions and the endoplasmic reticulum (ER) stress-relative mechanism. MEG3 expression levels were detected by qRT-PCR in both tumor tissues and adjacent non-tumor tissues from 28 ESCC patients. pcDNA3.1-MEG3 recombinant plasmids were constructed and transfected to EC109 cells. Cell growth was analyzed by CCK-8 assay. Cell apoptosis was analyzed by fluorescence microscope and Annexin V/PI assay. The protein expression was determined by western blot analysis. The results showed that MEG3 decreased significantly in ESCC tissues relative to adjacent normal tissues. pcDNA3.1-MEG3 plasmids were successfully constructed and the expression level of MEG3 significantly increased after MEG3 transfection to EC109 cells. Ectopic expression of MEG3 inhibited EC109 cell proliferation and induced apoptosis in vitro. MEG3 overexpression increased the expression of ER stress-related proteins (GRP78, IRE1, PERK, ATF6, CHOP and cleaved‐caspase‐3). Our results first demonstrate that MEG3 is downregulated in ESCC tissues. MEG3 was able to inhibit cell growth and induced apoptosis in EC109 cells, most probably via activation of the ER stress pathway. Introduction Esophageal cancer is a main cancer in China and the sixth leading cause of cancer-related death worldwide, with considerable geographical variations in incidence (1). Shantou is a high‐risk area of esophageal squamous cell carcinoma. The clinicopathological features of ESCC are different from the esophageal adenocarcinoma in Western countries (2). The molecular mechanisms of esophageal cancer remain unclear. According to encode project, ~80% of the human genome is transcribed to RNA, of which ~90% genome transcribes into non-coding RNA. These non‐coding RNAs are increasingly recognized and defined as endogenous regulatory RNA involved in maintenance of cellular homeostasis during growth, development and pathogenesis disease (3,4). Maternally expressed gene 3 (MEG3), which is an imprinted gene, located on chromosome 14q32. MEG3 was first identified as the ortholog of gene trap locus 2 (Gtl2) in mice (5). It is an imprinted gene expressed from the maternal allele with a length of ~1.6‐kb nucleotides. Although MEG3 is expressed in many human normal tissues, the loss of MEG3 expression has been found in various types of human tumors, including non-small cell lung cancer, hepatoma, gastric, prostate and other cancers (6‐9). Furthermore, MEG3 can inhibit cancer cell proliferation or induce apoptosis in vitro and in vivo by stimulating p53-dependent transcription (10-12). Endoplasmic reticulum (ER) is the site of protein synthesis, protein folding, maintainance of calcium homeostasis, synthesis of lipids and sterols (13). Various stimuli can disturb ER homeostasis and result in the accumulation of unfolded or misfolded proteins leading to pathological consequences, generally known as ‘ER stress’. Homeostatic regulation in the ER is under the control of three evolutionary conserved pathways: IRE1a‐XBP‐1, PERK‐eIF2a and ATF6. These pathways, also referred to as the unfolded protein response (UPR), are activated in response to ER stress (14). UPR promotes cell adapt stress. However, when the response is not sufficient and ER stress persists, UPR changes its function from acting to promote cellular survival to commit the cell to apoptosis through upregulation of pro‐apoptotic factors, such as CHOP and caspase‐4/12 (15). Long non-coding RNA MEG3 induces cell apoptosis in esophageal cancer through endoplasmic reticulum stress ZHEN‐LUN HUANG1, RUI‐PEI CHEN1, XIAO‐TAO ZHOU1, HAO‐LIAN ZHAN1, MIN‐MIN HU1, BIN LIU2, GUAN‐DI WU3 and LING‐FEI WU1 Departments of 1Gastroenterology, and 2Neurosurgery, Second Affiliated Hospital, Shantou University Medical College, Shantou, Guangdong 515041; 3First Clinical Medical College, Southern Medical University, Guangzhou, Guangdong 510515, P.R. China Received November 1, 2016; Accepted March 27, 2017 DOI: 10.3892/or.2017. 5568 Correspondence to: Professor Ling‐Fei Wu, Department of Gastroenterology, Second Affiliated Hospital, Shantou University Medical College, Shantou, Guangdong 515041, P.R. China E-mail: 1808435253@qq.com