MicroRNAs in liver disease

Abstract

MicroRNAs are small noncoding RNA molecules that regulate gene expression posttranscriptionally through complementary base pairing with thousands of messenger RNAs. They regulate diverse physiological, developmental, and pathophysiological processes. Recent studies have uncovered the contribution of microRNAs to the pathogenesis of many human diseases, including liver diseases. Moreover, microRNAs have been identified as biomarkers that can often be detected in the systemic circulation. We review the role of microRNAs in liver physiology and pathophysiology, focusing on viral hepatitis, liver fibrosis, and cancer. We also discuss microRNAs as diagnostic and prognostic markers and microRNA-based therapeutic approaches for liver disease.

Figure 1.

Timeline for studies of miRNAs in cancer. A PubMed search was conducted in June 2011 using the keywords “microRNA” or “miRNA” in titles or abstracts. A subsequent search was then restricted to liver or cancer.

Figure 2.

Steps in miRNA biogenesis. miRNAs are mainly transcribed by RNA polymerase II (RNAPII) into initial transcripts know as pre-miRNA, which are processed either via a canonical pathway, in which they are cropped by the Drosha-DGCR8 complex, or via the Mirtrons pathway, in which they are spliced to form hairpin-like miRNA precursors called pre-miRNAs. These precursors are exported from the nucleus to the cytoplasm in an Exportin-5-RanGTP—dependent manner. In the cytoplasm, they are processed by Dicer into an imperfect duplex comprising mature miRNAs and a complementary fragment called miRNA. The processed miRNAs are then loaded onto the RISC and are guided to their mRNA targets through interacting with various members of the Argonaute family such as Ago1–4 or Ago2 to form RISCs that are known as miRNP or miRISC. miRNA-mediated gene silencing or activation could be achieved through several identified mechanisms such as mRNA deadenylation, translational repression or translational activation, and possibly many more mechanisms yet to be discovered. Different base-paring combinations of miRNA-mRNA through their unique complementarities could result in gene silencing or gene activation.

Figure 3.

Roles of miRNAs in the pathogenesis of chronic liver disease and cancer. Cells in the liver, including hepatic stem and progenitor cells, hepatocytes, hepatic stellate cells, and immune cells, contribute to the progression of chronic liver disease to liver cancer. Factors such as HBV and HCV infection, obesity, and alcohol contribute to development of chronic liver disease, which can lead to fibrosis and cirrhosis and increase risk for HCC and intrahepatic cholangiocarcinoma (ICC). The figure shows miRNAs that are involved in different stages of liver disease.

Figure 4.

miRNAs participate in negative and positive feedback loops that regulate stem cell properties of cancer cells. (A) During normal development, the RNA-binding protein Lin28 is highly expressed in stem and progenitor cells. Lin28 blocks processing of let-7 miRNA precursor molecules into mature miRNAs to maintain expression of genes that promote self-renewal and proliferation. As progenitor cells differentiate, Lin28 expression decreases, which allows let-7 processing and increased production of mature let-7 miRNAs. These further inhibit Lin28, c-Myc, and several other cancer-associated genes. Similarly, c-Myc can promote self-renewal and proliferation in stem and progenitor cells, in part by inhibiting let-7 expression. Let-7 then represses the expression of genes involved in self-renewal, resulting in lineage commitment and terminal differentiation. In liver cancer cells, especially those with stem cell features, Lin28 and c-myc are highly activated, which results in an inactivation of let-7 and the maintenance of stem cell–like features of cancer cells, in a negative feedback loop. Let-7 can directly target Lin28, c-myc, K-Ras, HMGA2, Cyclin D1, and soluble collagens such as COL1A1 and COL1A2. (B) Signaling pathways such as Wnt–β-catenin, EpCAM, and miR-181 are activated in hepatic cancer stem cells. miR-181 promotes stem cell features of HCC cells by targeting CDX2 and GATA6 (hepatic transcriptional regulators of differentiation) and NLK (an inhibitor of Wnt–β-catenin signaling); this inhibition of NLK up-regulates transcription of EpCAM. Upon cleavage by TACE/PS-2, an intracellular ectodomain of EpCAM, referred as EpICD, translocates to the nucleus in a multiprotein complex that contains FHL2, β-catenin, and Lef-1 to induce transcription of genes that encode cyclin D1, c-Myc, and miR-181. This positive feedback loop promotes the stem cell-like characteristics of HCC cells.

Figure 5.

Origins of extracellular, circulating miRNA. Schematic summary of miRNA translocation from intracellular forms (free, complexed, and located in exosomes in multivesicular bodies) to extracellular forms. Three main mechanisms are believed to contribute to this as indicated. One mechanism is cell death (apoptosis or necrosis) with release of intracellular species such as protein-associated miRs (in RISC together with mRNA), in argonaute-miR complexes (miRNA-miRNPs), in exosomes, and as free miRNA that is quickly degraded or stabilized by association with plasma proteins such as high-density lipoproteins and albumin. Another mechanism is through secretion of exosomes by fusion of multivesicular bodies with the plasma membrane. Finally, exocytosis of miRNPs is believed to occur. Different miRNAs appear to participate in different complexes and to different degrees in exosomes and exosome-independent forms, indicating specific roles in intercellular signaling, such as through putative miRNA, miRNP, and exosome receptors.

Figure 6.

Size of human miRNA families and potential interactions with antimiRs. (A) Number of human miRNA families with 2 or more family members. (B) Number of all human miRNA families that can be targeted by antimiRs with lengths from 7 to 19 nucleotides.