Epigenetically silenced microRNAs in gastric cancer: Functional analysis and identification of their target genes

microRNAs (miRNAs), which are small non‑coding RNA molecules, can participate in diverse biological functions and act as oncogenes or tumor suppressors by inhibiting target gene expression. The alteration of miRNA expression is observed in many types of human cancers and has been implicated in carcinogenesis. Since miRNAs have been known to be downregulated in most cancer types, there is growing evidence that several miRNAs are downregulated by DNA hypermethylation. Here, we determined that MIR219.2, MIR663b and MIR1237 were transcriptionally silenced by DNA hypermethylation in human gastric cancer cell lines. Moreover, we demonstrated the functional roles of these epigenetically silenced miRNAs by ectopically expressing them in gastric cancer cells, which caused the suppression of growth and proliferation. In addition, wound closure, cell migration, and invasion were significantly reduced in AGS cells following transfection with MIR219.2, MIR663b or MIR1237 mimics. Notably, epithelial-to-mesenchymal transition (EMT)-associated proteins were decreased in response to ectopic expression of these miRNAs, supporting the notion that these miRNAs have a tumor-suppressive effect in gastric cancer. We finally predicted the targets of these miRNAs and identified several candidate genes, the expression levels of which were significantly downregulated by ectopic expression of MIR219.2, MIR663b or MIR1237 mimics in the gastric cancer cell lines. Our study provides strong evidence that these miRNAs are transcriptionally regulated by DNA methylation in gastric cancer and have tumor-suppressive roles by decreasing the mesenchymal traits in cancer as well as by targeting cancer-associated genes.