MicroRNA-122 (miR-122), which accounts for 70% of the liver's total miRNAs, plays a pivotal role in the liver. However, its intrinsic physiological roles remain largely undetermined. We demonstrated that mice lacking the gene encoding miR-122a (Mir122a) are viable but develop temporally controlled steatohepatitis, fibrosis, and hepatocellular carcinoma (HCC). These mice exhibited a striking disparity in HCC incidence based on sex, with a male-to-female ratio of 3.9:1, which recapitulates the disease incidence in humans. Impaired expression of microsomal triglyceride transfer protein (MTTP) contributed to steatosis, which was reversed by in vivo restoration of Mttp expression. We found that hepatic fibrosis onset can be partially attributed to the action of a miR-122a target, the Klf6 transcript. In addition, Mir122a -/-livers exhibited disruptions in a range of pathways, many of which closely resemble the disruptions found in human HCC. Importantly, the reexpression of miR-122a reduced disease manifestation and tumor incidence in Mir122a -/-mice. This study demonstrates that mice with a targeted deletion of the Mir122a gene possess several key phenotypes of human liver diseases, which provides a rationale for the development of a unique therapy for the treatment of chronic liver disease and HCC.

Our study suggests that miR-122, a tumor suppressor microRNA affecting hepatocellular carcinoma intrahepatic metastasis by angiogenesis suppression, exerts some of its action via regulation of ADAM17. Restoration of miR-122 has a far-reaching effect on the cell. Using the concomitant down-regulation of its targets, including ADAM17, a rational therapeutic strategy based on miR-122 may prove to be beneficial for patients with hepatocellular carcinoma.

G9a is a mammalian histone methyltransferase that contributes to the epigenetic silencing of tumor suppressor genes. Emerging evidence suggests that G9a is required to maintain the malignant phenotype, but the role of G9a function in mediating tumor metastasis has not been explored. Here, we show that G9a is expressed in aggressive lung cancer cells, and its elevated expression correlates with poor prognosis. RNAi-mediated knockdown of G9a in highly invasive lung cancer cells inhibited cell migration and invasion in vitro and metastasis in vivo. Conversely, ectopic G9a expression in weakly invasive lung cancer cells increased motility and metastasis. Mechanistic investigations suggested that repression of the cell adhesion molecule Ep-CAM mediated the effects of G9a. First, RNAi-mediated knockdown of Ep-CAM partially relieved metastasis suppression imposed by G9a suppression. Second, an inverse correlation between G9a and Ep-CAM expression existed in primary lung cancer. Third, Ep-CAM repression was associated with promoter methylation and an enrichment for dimethylated histone H3K9. G9a knockdown reduced the levels of H3K9 dimethylation and decreased the recruitment of the transcriptional cofactors HP1, DNMT1, and HDAC1 to the Ep-CAM promoter. Our findings establish a functional contribution of G9a overexpression with concomitant dysregulation of epigenetic pathways in lung cancer progression. Cancer Res; 70(20); 7830-40. ©2010 AACR.