MicroRNAs (miRNAs) are evolutionary conserved small non-coding RNAs that regulate gene expression by mediating post-transcriptional silencing of target genes. Since miRNAs are involved in fine-tuning of physiological responses, they have become of interest for diagnosis and therapy of a number of diseases. Moreover, the role of dysregulated miRNAs in maintaining the malignant phenotype has profound implications for cancer therapy. We will review the best defined cellular miRNAs and changes in their expression profile in hepatocellular carcinoma (HCC). Cellular miRNAs can also be released into the circulation, and these miRNAs are detected in most body fluids. Circulating miRNAs are associated with HCC and are possible biomarkers. Finally, by affecting several clinically relevant targets, artificially increasing or decreasing the expression level of a given miRNA offers fascinating therapeutic perspectives. We will therefore highlight recent developments in miRNA-based gene therapy with a focus on their therapeutic potential for HCC.

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are drug efflux pumps responsible for the multidrug resistance phenotype causing hepatocellular carcinoma (HCC) treatment failure. Here we studied the expression of 15 ABC transporters relevant for multidrug resistance in 19 paired HCC patient samples (16 untreated, 3 treated by chemotherapeutics). Twelve ABC transporters showed up-regulation in HCC compared with adjacent healthy liver. These include ABCA2, ABCB1, ABCB6, ABCC1, ABCC2, ABCC3, ABCC4, ABCC5, ABCC10, ABCC11, ABCC12, and ABCE1. The expression profile and function of some of these transporters have not been associated with HCC thus far. Because cellular microRNAs (miRNAs) are involved in posttranscriptional gene silencing, we hypothesized that regulation of ABC expression in HCC might be mediated by miRNAs. To study this, miRNAs were profiled and dysregulation of 90 miRNAs was shown in HCC compared with healthy liver, including up-regulation of 11 and down-regulation of 79. miRNA target sites in ABC genes were bioinformatically predicted and experimentally verified in vitro using luciferase reporter assays. In total, 13 cellular miRNAs were confirmed that target ABCA1, ABCC1, ABCC5, ABCC10, and ABCE1 genes and mediate changes in gene expression. Correlation analysis between ABC and miRNA expression in individual patients revealed an inverse relationship, providing an indication for miRNA regulation of ABC genes in HCC. Conclusion: Up-regulation of ABC transporters in HCC occurs prior to chemotherapeutic treatment and is associated with miRNA down-regulation. Up-regulation of five ABC genes appears to be mediated by 13 cellular miRNAs in HCC patient samples. miRNA-based gene therapy may be a novel and promising way to affect the ABC profile and overcome clinical multidrug resistance. (HEPATOLOGY 2012;55:821-832) H epatocellular carcinoma (HCC) is the fifth most common type of cancer worldwide. With a 5-year survival of less than 5%, 1 HCC remains one of the most fatal cancers, and few treatments have proven to be effective. Major pitfalls are late diagnosis, tumor recurrence, and resistance to chemotherapeutic treatment. This is caused by a phenomenon called multidrug resistance, mediated by high expression of adenosine triphosphate (ATP)-binding cassette (ABC) transporter family members that decrease the intracellular concentration of chemotherapeutic agents. [2][3][4][5][6] There is limited information in the literature on the expression profile of ABC genes in HCC. For example, ABCB1 (MDR1) 7,8 and ABCC3 (MRP3) 9 have been shown to be up-regulated in HCC of undetermined treatment status and a high expression of ABCC1 (MRP1) has been associated with an aggressive HCC phenotype in untreated