Hepatocellular carcinoma is the third leading cause of cancer mortality worldwide, but the molecular mechanisms in tumorigenesis remain largely unknown. Previously, a DEAD-box protein DDX20, a component of microRNA-containing ribonucleoprotein complexes, was identified as a liver tumor suppressor candidate in an oncogenomics-based in vivo RNAi screen. However, the molecular mechanisms were unknown. Here, we show that deficiency of DDX20 results in the enhancement of NF-kappa B activity, a crucial intracellular signaling pathway closely linked with hepatocarcinogenesis. While DDX20 normally suppresses NF-kappa B activity by regulating NF-kappa B-suppressing miRNA-140 function, this suppressive effect was lost in DDX20-deficient cells. The impairment of miRNA function due to DDX20 deficiency appears to be miRNA species-specific at the point of loading miRNAs into the RNA-induced silencing complex. These results indicate that DDX20 deficiency enhances NF-kappa B activity by impairing the NF-kappa B-suppressive action of microRNAs, and suggest that dysregulation of the microRNA machinery components may also be involved in pathogenesis in various human diseases. (C) 2012 Elsevier Inc. All rights reserved.