MicroRNA-218 Inhibits Cell Cycle Progression and Promotes Apoptosis in Colon Cancer by Downregulating BMI1 Polycomb Ring Finger Oncogene

Deregulated miRNAs participate in colorectal carcinogenesis. In this study, miR-218 was found to be downregulated in human colorectal cancer (CRC) by miRNA profile assay. miR-218 was silenced or downregulated in all five colon cancer cells (Caco2, HT29, SW620, HCT116 and LoVo) relative to normal colon tissues. miR-218 expression was significantly lower in 46 CRC tumor tissues compared with their adjacent normal tissues (P < 0.001). Potential target genes of miR-218 were predicted and BMI1 polycomb ring finger oncogene (BMI-1), a polycomb ring finger oncogene, was identified as one of the potential targets. Upregulation of BMI-1 was detected in CRC tumors compared with adjacent normal tissues (P < 0.001) and in all five colon cancer cell lines. Transfection of miR-218 in colon cancer cell lines (HCT116, HT29) significantly reduced luciferase activity of the wild-type construct of BMI-1 3' untranslated region (3'UTR) (P < 0.001), whereas this effect was not seen in the construct with mutant BMI-1 3'UTR, indicating a direct and specific interaction of miR-218 with BMI-1. Ectopic expression of miR-218 in HCT116 and HT29 cells suppressed BMI-1 mRNA and protein expression. In addition, miR-218 suppressed protein expression of BMI-1 downstream targets of cyclin-dependent kinase 4, a cell cycle regulator, while upregulating protein expression of p53. We further revealed that miR-218 induced apoptosis (P < 0.01), inhibited cell proliferation (P < 0.05) and promoted cell cycle arrest in the G2 phase (P < 0.01). In conclusion, miR-218 plays a pivotal role in CRC development through inhibiting cell proliferation and cycle progression and promoting apoptosis by downregulating BMI-1.