Resistance to TGF-β-induced growth repression is prevalent in various cancer cells, but the underlying mechanisms remain unclear. In this study, we showed that activation of TGF-β signaling caused Sma- and Mad-related family (Smad) 2 and Smad3 to bind directly to the promoter region of miR-195, and then activated miR-195 transcription in normal hepatocytes. Conversely, miR-195 inhibited the expression of Smad7 by binding to its 3'-UTR, thereby strengthening TGF-β-Smad signaling. These data identify a novel TGF-β-miR-195 positive regulatory circuitry in normal hepatocytes. Further investigation revealed that HDAC1, a histone deacetylase that was abnormally overexpressed in hepatocellular carcinoma, could bind to the miR-195 promoter via Smad3 and cause hypoacetylation in the histones associated with the miR-195 promoter in hepatoma cells. This resulted in transcriptional repression of miR-195 and, subsequently, disruption of the TGF-β-miR-195 regulatory loop and evasion of TGF-β-mediated growth inhibition. Moreover, silencing HDAC1 in hepatoma cells restored TGF-β-mediated growth suppression, but this effect was attenuated if miR-195 expression decreased. These findings suggest that HDAC1-induced miR-195 down-regulation is an important mechanism for tumor cells to resist the cytostatic activity of TGF-β, and highlight the importance of TGF-β-Smad2/3-miR-195-Smad7 circuitry in preventing uncontrolled cell proliferation.-Wang, R., Fu, T., You, K., Li, S., Zhao, N., Yang, J., Zhuang, S.-M. Identification of a TGF-β-miR-195 positive feedback loop in hepatocytes and its deregulation in hepatoma cells.
Keywords: HDAC1; TGF-β signaling; hepatocellular carcinoma; miR-195; noncoding RNA.