MicroRNAs (miRNAs) are evolutionary conserved small RNAs that post‐transcriptionally regulate the expression of target genes. To date, the role of miRNAs in liver development is not fully understood. By using an experimental model that allows the induced and controlled differentiation of mouse fetal hepatoblasts (MFHs) into mature hepatocytes, we identified miR‐148a as a hepatospecific miRNA highly expressed in adult liver. The main finding of this study revealed that miR‐148a was critical for hepatic differentiation through the direct targeting of DNA methyltransferase (DNMT) 1, a major enzyme responsible for epigenetic silencing, thereby allowing the promotion of the “adult liver” phenotype. It was also confirmed that the reduction of DNMT1 by RNA interference significantly promoted the expression of the major hepatic biomarkers. In addition to the essential role of miR‐148a in hepatocyte maturation, we identified its beneficial effect through the repression of hepatocellular carcinoma (HCC) cell malignancy. miR‐148a expression was frequently down‐regulated in biopsies of HCC patients as well as in mouse and human HCC cell lines. Overexpressing miR‐148a led to an enhancement of albumin production and a drastic inhibition of the invasive properties of HCC cells, whereas miR‐148a silencing had the opposite consequences. Finally, we showed that miR‐148a exerted its tumor‐suppressive effect by regulating the c‐Met oncogene, regardless of the DNMT1 expression level. Conclusion: miR‐148a is essential for the physiology of the liver because it promotes the hepatospecific phenotype and acts as a tumor suppressor. Most important, this report is the first to demonstrate a functional role for a specific miRNA in liver development through regulation of the DNMT1 enzyme. (Hepatology 2013;53:1153–1165)

Genetic alterations and deregulation of the miRNA biogenesis pathway components have been reported in human tumors. Tissue‐specific deletion of the Dicer gene, which encodes an essential miRNA processing enzyme, promotes carcinogenesis in animal models. These features indicate that aberrant miRNA biogenesis components are directly associated with cancer. For the present study, we conducted quantitative RT‐PCR of 14 genes that are related to the miRNA biogenesis pathway in 47 paired samples of primary hepatocellular carcinoma (HCC) and matched non‐cancerous liver. Expression of seven genes (Dgcr8, p68, p72, Dicer, Ago3, Ago4 and Piwil4) was significantly decreased in primary HCC, especially in non‐viral HCC subtypes, compared to the non‐cancerous liver. Combinations of decreased expression of the miRNA biogenesis components in non‐cancerous liver were related to cigarette smoking, alcohol intake and diabetes, which are known to be risk factors for HCC, and were also associated with the occurrence of multicentric tumors. Reduction of two of these genes (Dicer and p68) in HCC was associated with poor prognosis. Trimethylation of histone H3 lysine 27 in the promoters is implicated in the deregulation of these miRNA‐biogenesis‐related genes in non‐HBV genome integrated HCC cell lines. In conclusion, deregulation of the miRNA biogenesis pathway components is frequently observed in non‐viral‐associated HCC and is linked to etiological risk factors and poor prognosis. Our study further showed that epigenetic regulation could be implicated in the deregulation of these genes during hepatocarcinogenesis.

Chromosomal abnormalities are good guideposts when hunting for cancer-related genes. We analyzed copy number alterations of 163 primary gastric cancers using array-based comparative genomic hybridization and simultaneously performed a genome-wide integrated analysis of copy number and gene expression using microarray data for 58 tumors. We showed that chromosome 6p21 amplification frequently occurred secondary to ERBB2 amplification, was associated with poorer prognosis and caused overexpression of half of the genes mapped. A comprehensive small interfering RNA knockdown of 58 genes overexpressed in tumors identified 32 genes that reduced gastric cancer cell growth. Enforced expression of 16 of these genes promoted cell growth in vitro, and six genes showing more than two-fold activity conferred tumor-forming ability in vivo. Among these six candidates, GLO1, encoding a detoxifying enzyme glyoxalase I (GLO1), exhibited the strongest tumor-forming activity. Coexpression of other genes with GLO1 enhanced growth-stimulating activity. A GLO1 inhibitor, S-p-bromobenzyl glutathione cyclopentyl diester, inhibited the growth of two-thirds of 24 gastric cancer cell lines examined. The efficacy was found to be associated with the mRNA expression ratio of GLO1 to GLO2, encoding glyoxalase II (GLO2), another constituent of the glyoxalase system. GLO1 downregulation affected cell growth through inactivating central carbon metabolism and reduced the transcriptional activities of nuclear factor kappa B and activator protein-1. Our study demonstrates that GLO1 is a novel metabolic oncogene of the 6p21 amplicon, which promotes tumor growth and aberrant transcriptional signals via regulating cellular metabolic activities for energy production and could be a potential therapeutic target in gastric cancer.