The human genome is replete with long non-coding RNAs (lncRNA), many of which are transcribed and likely to have a functional role. Microarray analysis of >23 000 lncRNAs revealed downregulation of 712 (∼3%) lncRNA in malignant hepatocytes, among which maternally expressed gene 3 (MEG3) was downregulated by 210-fold relative to expression in non-malignant hepatocytes. MEG3 expression was markedly reduced in four human hepatocellular cancer (HCC) cell lines compared with normal hepatocytes by real-time PCR. RNA in situ hybridization showed intense cytoplasmic expression of MEG3 in non-neoplastic liver with absent or very weak expression in HCC tissues. Enforced expression of MEG3 in HCC cells significantly decreased both anchorage-dependent and -independent cell growth, and induced apoptosis. MEG3 promoter hypermethylation was identified by methylation-specific PCR and MEG3 expression was increased with inhibition of methylation with either 5-Aza-2-Deoxycytidine, or siRNA to DNA Methyltransferase (DNMT) 1 and 3b in HCC cells. MiRNA-dependent regulation of MEG3 expression was studied by evaluating the involvement of miR-29, which can modulate DNMT 1 and 3. Overexpression of mir-29a increased expression of MEG3. GTL2, the murine homolog of MEG3, was reduced in liver tissues from hepatocyte-specific miR-29a/b1 knock-out mice compared with wild-type controls. These data show that methylation-dependent tissue-specific regulation of the lncRNA MEG3 by miR-29a may contribute to HCC growth and highlight the inter-relationship between two classes of non-coding RNA, miRNAs and lncRNAs, and epigenetic regulation of gene expression.

Hepatocellular carcinoma (HCC) is characterized by a propensity for multifocality, growth by local spread, and dysregulation of multiple signaling pathways. These features may be determined by the tumoral microenvironment. The potential of tumor cells to modulate HCC growth and behavior by secreted proteins has been extensively studied. In contrast, the potential for genetic modulation is poorly understood. We investigated the role and involvement of tumor-derived nanovesicles capable of altering gene expression and characterized their ability to modulate cell signaling and biological effects in other cells. We show that HCC cells can produce nanovesicles and exosomes that differ in both RNA and protein content from their cells of origin. These can be taken up and internalized by other cells and can transmit a functional transgene. The microRNA (miRNA) content of these exosomes was examined, and a subset highly enriched within exosomes was identified. A combinatorial approach to identify potential targets identified transforming growth factor β activated kinase-1 (TAK1) as the most likely candidate pathway that could be modulated by these miRNAs. Loss of TAK1 has been implicated in hepatocarcinogenesis and is a biologically plausible target for intercellular modulation. We show that HCC cell-derived exosomes can modulate TAK1 expression and associated signaling and enhance transformed cell growth in recipient cells. Conclusion : Exosome-mediated miRNA transfer is an important mechanism of intercellular communication in HCC cells. These observations identify a unique intercellular mechanism that could potentially contribute to local spread, intrahepatic metastases, or multifocal growth in HCC. (Hepatology 2011;)

Hepatocellular cancers (HCC) are highly resistant to chemotherapy. TGFβ has been associated with chemoresistance in some human cancers but the mechanisms involved are unknown. We explored how TGFβ might contribute to altered responses to therapy by assessing the involvement and mechanistic contribution of extracellular vesicle long non‐coding RNA (lncRNA) in mediating TGFβ‐dependent chemoresistance. TGFβ reduced the sensitivity of HCC cells to sorafenib or doxorubicin and altered the release of both extracellular vesicles and of selected lncRNA within these vesicles. Amongst these, lincRNA‐ROR (linc‐ROR), a stress‐responsive lncRNA was highly expressed in HCC cells and enriched within extracellular vesicles derived from tumor cells. Incubation with HCC‐derived extracellular vesicles increased linc‐ROR expression and reduced chemotherapy‐induced cell death in recipient cells. Sorafenib increased linc‐ROR expression in both tumor cells and extracellular vesicles, whereas siRNA to linc‐ROR increased chemotherapy‐induced apoptosis and cytotoxicity. Tumor‐initiating cells that express CD133 have an increased resistance to therapy. TGFβ increased expression of CD133+ cells and colony growth in limiting dilution assays, both of which were attenuated by linc‐ROR knockdown. These data provide mechanistic insights into primary chemoresistance in HCC by showing that: (a) TGFβ selectively enriches linc‐RoR within extracellular vesicles, which has a potential role in intercellular signaling in response to TGFβ; (b) expression and enrichment of linc‐ROR during chemotherapeutic stress plays a functional role in chemoresistance; and (c) the effects of TGFβ on chemoresistance in HCC may involve linc‐RoR‐dependent effects on tumor‐initiating cells. These findings implicate extracellular vesicle lncRNA as mediators of the chemotherapeutic response, and support targeting linc‐ROR to enhance chemosensitivity in HCC.