Background: microRNA-122 (miR-122) is the most abundant and specific miRNA in the liver. It acts as an important tumor suppressor in hepatocellular carcinoma (HCC) through regulating its target genes, but details of its own regulation are largely unknown. Farnesoid X receptor (FXR), a transcription factor with multiple functions, plays an important role in protecting against liver carcinogenesis, but it is unclear whether the anti-HCC effect of FXR is involved in the regulation of miR-122.
Methods: The levels of miR-122 and FXR in HCC tissues and cell lines were examined by quantitative real-time PCR (qRT-PCR). qRT-PCR was also used to detect the expression of miR-122 target genes at mRNA level, while Western blotting was used to analyze that of their protein products. The effect of FXR on the transcriptional activity of miR-122 promoter was evaluated by a luciferase reporter assay. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay were performed to identify the FXR binding site within miR-122 promoter region. The cell proliferation was analyzed by a CCK-8 assay. The influence of FXR on tumor growth and miR-122 expression in vivo was monitored using HCC xenografts in nude mice.
Results: The expression of FXR was positively correlated with that of miR-122 in HCC tissues and cell lines. Activation of FXR in HCC cells upregulated miR-122 expression and in turn downregulated the expression of miR-122 target genes including insulin-like growth factor-1 receptor and cyclin G1. FXR bound directly to the DR2 element (-338 to -325) in miR-122 promoter region, and enhanced the promoter's transcriptional activity. Functional experiments showed that the FXR-mediated upregulation of miR-122 suppressed the proliferation of HCC cells in vitro and the growth of HCC xenografts in vivo.
Conclusions: miR-122 is a novel target gene of FXR, and the upregulation of miR-122 by FXR represses the growth of HCC cells, suggesting that FXR may serve as a key transcriptional regulator for manipulating miR-122 expression, and the FXR/miR-122 pathway may therefore be a novel target for the treatment of HCC.