Distant migration and invasion is the main contributor to the death of cancer patients and miRNAs have been implicated in these processes. In the present study, we identified the role of microRNA-21 (miR-21) in hepatocellular carcinoma (HCC) migration and invasion and determined its underlying regulatory mechanism. miR-21 was significantly upregulated in HCC tissues and cell lines, compared with adjacent non-tumor tissues and normal hepatic cells. miR-21 upregulation was associated with the capacity of tumor migration and invasion in HCC. The expression level of miR-21 was inversely correlated with the protein expression level of a previously identified target gene, programmed cell death 4 (PDCD4). In HepG2 cells, inhibition of miR-21 expression repressed cell migration and invasion by upregulating both mRNA and protein levels of PDCD4 and downregulating key downstream signaling pathway molecules, including phospho-c-Jun, matrix metalloproteinases (MMP)-2 and MMP-9. Moreover, activation protein 1 (AP-1) could directly activate miR-21 transcription. Taken together, these results provide evidence that miR-21 promotes migration and invasion in HCC through the miR-21-PDCD4-AP-1 feedback loop, suggesting that targeting the miR-21-PDCD4-AP-1 loop may represent a promising strategy in the management of HCC.

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