The role of the miR-31/FIH1 pathway in TGF-β-induced liver fibrosis

The miRNAs are small, non-coding RNAs that regulate various biological processes, including liver fibrosis. Hepatic stellate cells (HSCs) play a central role in the pathogenesis of liver fibrosis. By microarray profiling and real-time PCR, we noted that miR-31 expression in HSCs from rats, mice and humans was significantly increased during HSC activation in culture. Overall, miR-31 expression levels were unchanged in the whole-liver RNA extracts from fibrotic rat and human samples. Nevertheless, we found that miR-31 was particularly up-regulated in HSCs but not in hepatocytes during fibrogenesis. Thus, we hypothesized that miR-31 may mediate liver fibrosis. In the present study, we found that inhibition of miR-31 expression significantly inhibited HSC activation, whereas its over-expression obviously promoted HSC activation. Moreover, over-expression of miR-31 promoted HSC migration by enhancing matrix metalloproteinase (MMP)-2 expression whereas inhibition of miR-31 has an opposite effect. The biological function of miR-31 during HSC activation might be through targeting FIH1, a suppressor of hypoxia-inducible factor (HIF-1), because a knockdown of FIH1 by shRNA could mimic the effects of miR-31. In addition, primary rat HSCs were isolated and treated with different cytokines, such as transforming growth factor β (TGF-β), vascular endothelial growth factor and platelet-derived growth factor-BB, to evaluate upstream regulators of miR-31. We found that only TGF-β, a pivotal regulator in liver fibrosis, remarkably increased miR-31 expression in HSCs. And the effects of TGF-β on HSCs can be partially counteracted by inhibition of miR-31. In addition, chromatin immunoprecipitation experiments and the luciferase reporter assay demonstrated that Smad3, a major TGF-β-downstream transcription factor, stimulated the transcription activity of miR-31 by binding directly to miR-31's promoter. In conclusion, the miR-31/FIH1 pathway associates with liver fibrosis, perhaps by participation in the TGF-β/Smad3 signalling of HSCs.