The regulation network consisting of microRNAs (miRNAs) and their target genes remains largely elusive in hepatocellular carcinoma (HCC), especially the reciprocal loop between specific miRNAs and the miRNA processing machinery. In this study, we found that miR-99a was remarkably decreased in 111 of 152 (73.03%) primary HCC tissues and low-level expression of miR-99a was correlated with low tumor differentiation (P=0.001), liver cirrhosis (P=0.015), poor tumor-free survival (P=0.004) and overall survival (P=0.006) for HCC patients. By restoration of miR-99a, the HCC growth could be considerably inhibited both in vitro and in vivo. Subsequently, Argonaute-2 (Ago2), a central component of RNA-induced silencing complex, was found to be directly regulated by miR-99a via translational repression. Overexpression of Ago2 could partly impair the inhibitory effect of miR-99a on HCC cells in vitro. Then, we demonstrated that Ago2 was upregulated in HCC tissues at both RNA and protein levels and the expression of AGO2 protein and miR-99a was negatively correlated within detected HCC tissues (r=−0.727, P=0.004). Interestingly, the tumorigenicity of Ago2-knockdown HCC cells was severely impaired (4/10 vs 10/10, P<0.05), and this was in contrast to the miR-99a-overexpressing HCC cells. Functionally, the increased AGO2 protein could specifically facilitate oncogenic miR-21 to repress its targeted gene phosphatase and tensin homolog (Pten) in HCC, whereas leave the regulatory capacity of let-7a on its targeted oncogenes almost unaltered. In summary, our study has revealed a novel pathway for the tumor suppressor miR-99a to control tumor growth in HCC, via its downstream signaling of AGO2/miR-21/PTEN. In addition, this study provides potential strategies for HCC therapy by reintroduction of miRNA suppressors.

Hepatocarcinogenesis is a complex process involving chronic liver injury, inflammation, unregulated wound healing, subsequent fibrosis and carcinogenesis. To decipher the molecular mechanism underlying transition from chronic liver injury to dysplasia, we investigated the oncogenic role of gankyrin (PSMD10 or p28GANK) during malignant transformation in a transgenic mouse model. Here, we find that gankyrin increased in patients with cirrhosis. In addition to more severe liver fibrosis and tumorigenesis after DEN plus CCl4 treatment, hepatocyte-specific gankyrin-overexpressing mice (gankyrinhep) exhibited malignant transformation from liver fibrosis to tumors even under single CCl4 administration, whereas wild-type mice merely experienced fibrosis. Consistently, enhanced hepatic injury, severe inflammation and strengthened compensatory proliferation occurred in gankyrinhep mice during CCl4 performance. This correlated with augmented expressions of cell cycle-related genes and abnormal activation of Rac1/c-jun N-terminal kinase (JNK). Pharmacological inhibition of the Rac1/JNK pathway attenuated hepatic fibrosis and prevented CCl4-induced carcinogenesis in gankyrinhep mice. Together, these findings suggest that gankyrin promotes liver fibrosis/cirrhosis progression into hepatocarcinoma relying on a persistent liver injury and inflammatory microenvironment. Blockade of Rac1/JNK activation impeded gankyrin-mediated hepatocytic malignant transformation, indicating the combined inhibition of gankyrin and Rac1/JNK as a potential prevention mechanism for cirrhosis transition.