MicroRNA, an endogenous noncoding RNA modulating gene expression, is a key 
molecule that by its dysregulation plays roles in inflammatory-driven 
carcinogenesis. This study aimed to investigate the role of oncomiR miR-21 and 
its target, the programmed cell death 4 (PDCD4) in tumor growth and metastasis 
of the liver fluke Opisthorchis viverrini-associated cholangiocarcinoma (CCA). 
The expression levels of miR-21 and PDCD4 were analyzed using the TaqMan miRNA 
expression assay and immunohistochemistry in liver tissues of both O. viverrini 
plus N-nitrosodimethylamine (NDMA)-treated hamsters and human CCA samples (n=23 
cases). The functional assay for miR-21 was performed in CCA cell lines by the 
anti-miR-21 and pre-miR-21 transfection procedures. The peak of miR-21 levels 
were reached at 2 (hyperplastic lesions) and 6 (CCA) months of the O. viverrini 
plus NDMA-induced group and had a reverse response with its target PDCD4 
proteins. In human CCA, miR-21 was overexpressed in tumor tissues when compared 
with nontumor tissues (P=0.0034) and had a negative correlation with PDCD4 
protein (P=0.026). It was also found that high expression of miR-21 was 
significantly correlated with shorter survival (P<0.05) and lymph node 
metastasis (P=0.037) of CCA patients. Transient transfection of pre-miR-21 
reduced the PDCD4 level and resulted in an increase of M213 CCA cell growth and 
wound-induced migration ability. These results indicated that miR-21 plays a 
role in the carcinogenesis and metastasis of O. viverrini-associated CCA by 
suppressing the function of PDCD4. Modulation of aberrantly expressed miR-21 may 
be a useful strategy to inhibit tumor cell phenotypes or improve response to 
chemotherapy.