Pri-miRNAs are cleaved to precursor miRNAs (pre-miRNAs) of about 50–150 nucleotides by Drosha, an endoribonuclease III (RNAse III), and its cofactor RNA-binding protein Pasha (DGCR8). The pre-miRNAs are then exported to the cytoplasm by exportin 5.

This is further excised to double-stranded duplices of 20–23 nucleotides by Dicer, an RNAse III enzyme. The miRNA duplex later separates into single-stranded mature miRNA, and incorporates into the RNA-induced silencing complex (RISC), which is composed of Argonaute proteins. This complex binds to the 3′-untranslated region (3′-UTR) of its target transcript Vemurafenib and negatively regulates protein translation by a mechanism that depends on the complementarity between the miRNA and target messenger RNA. Partial complementarity results in translational repression, while complete complementarity triggers mRNA degradation. An important feature of miRNA is that a single miRNA can regulate multiple target

mRNAs. This pleiotropic property enables miRNAs to exert wide control see more on a network of genes. It has been demonstrated that overexpression of a single miRNA can downregulate over 100 mRNAs.5 Further, bioinformatic analysis predicts miRNAs can affect up to 30% of all human genes.6 It is therefore not surprising that miRNAs are involved in diverse physiological and developmental processes. These range from cell survival, differentiation, responses to external

stress and morphogenesis. In cancer, an imbalance of miRNA expression was first described in B-cell chronic lymphocytic leukemia,7 and by now virtually all examined tumor types display abnormal miRNA expression patterns. Studies on cancer miRNA have shown that they are often downregulated in tumor tissues, irrespective of cell type.8 This general phenomenon may be exemplified by the observation of common downregulation of Dicer, a key enzyme in the miRNA biogenesis, in many human neoplasms, including hepatocellular carcinoma (HCC),9 ovarian cancer,10 gastric cancer,11 and lung adenocarcinoma.12 In addition, reduced Dicer expression has been shown to correlate with shorter postoperative survival of patients with non-small-cell lung carcinoma.13 A study correlating the medchemexpress expression of precursor and mature miRNAs further showed that a vast number of miRNAs was transcribed but not processed to mature forms in cancer cell lines.14 Taken together, these findings seem to indicate that defective miRNA processing promotes transformation of normal cells to cancer, and that the miRNAome, as a whole, plays a critical role in tumor suppression. This review will focus on the miRNA dysregulation in HCC. The first part summarizes the involvement of miRNAs in the pathogenicity of HCC risk factors.