Abstract:     Background and purpose: MicroRNA (miRNA, miR) plays an important regulatory role in cancer. miR-222 is reported to be up-regulated in various tumors, but its role in renal cell carcinoma (RCC) remains unclear. In this study, we detected the expression of miR-222 in both RCC and adjacent tissue samples. The aim of this study was to investigate the role of miR-222 in RCC. Methods: The expression levels of miR-222 in RCC tissue samples were quantified by quantitative real-time polymerase chain reaction (qRT-PCR). DDIT4 and LC3-Ⅱ protein expressions were determined by Western blot. Dual luciferase assay was performed to verify the target of miR-222. EGFP-LC3 microscopy assay was performed to assess autophagy. Results: Results from qRT-PCR showed that the expression of miR-222 was up-regulated in RCC tissues. Knockdown of miR-222 with specific antagomiR decreased the cell viability of 786-O cells, whereas overexpression of miR-222 increased the cell viability (P<0.01). The levels of DDIT4 were upregulated in 786-O cells transfected with miR-222 antagomiR, whereas overexpression of miR-222 induced the downregulation of DDIT4 expression. Data from dual luciferase assay indicated that miR-222 directly targeted the expression of DDIT4. Consistently, the expression of DDIT4 in RCC tissues was down-regulated compared with adjacent tissues. Knockdown of miR-222 in 786-O cells induced a significant increase of autophagosome formation and LC3 lipidation. These results supported that miR-222 could inhibit autophagy in RCC cells, which may affect the clinical characteristcs of RCC. Conclusion: miR-222 is up-regulated in RCC and can inhibit the autophagy of RCC cells through downregulating the expression of DDIT4.

Key words:  miR-222, Renal cell carcinoma, DDIT4, Autophagy