Positive and negative modulation of viral and cellular mRNAs by liver-specific microRNA miR-122

microRNAs (miRNAs) are small RNAs that in general down-regulate the intracellular abundance and translation of target mRNAs. We noted that sequestration of liver-specific miR-122 by modified antisense oligonucleotides resulted in a dramatic loss of hepatitis C virus (HCV) RNA in cultured human liver cells. A binding site for miR-122 was predicted to reside close to the 5' end of the viral genome, and its functionality was tested by mutational analyses of the miRNA-binding site in viral RNA, resulting in reduced intracellular viral RNA abundance. Importantly, ectopic expression of miR-122 molecules that contained compensatory mutations restored viral RNA abundance, revealing a genetic interaction between miR-122 and the viral RNA genome. Studies with replication-defective viral RNAs demonstrated that miR-122 affected mRNA abundance by positively modulating RNA replication. In contrast, interaction of miR-122 with the 3'-noncoding region (3'NCR) of the cellular mRNA encoding the cationic amino acid transporter CAT-1 resulted in the down-regulation of CAT-1 protein abundance. These findings provide evidence that a specific miRNA can regulate distinct target mRNAs in both a positive and negative fashion. The positive role of miR-122 in viral replication suggests that this miRNA could be targeted for antiviral therapy.

N-myc translation is initiated via an internal ribosome entry segment that displays enhanced activity in neuronal cells

Eukaryotic translation can be initiated either by a cap-dependent mechanism or by internal ribosome entry, a process by which ribosomes are directly recruited to structured regions of mRNA upstream of the initiation codon. We analysed the 5' untranslated region (UTR) of the proto-oncogene N-myc, and demonstrated by transfections in a dicistronic vector system that it contains a potent internal ribosome entry segment (IRES). The IRES is similar in length to the c-myc IRES and the activities of these IRESs are comparable in non-neuronal cells. Transfections were also carried out in cell lines derived from neuroblastomas, in which N-myc is expressed, and in a neuronal precursor cell line. In these cells the N-myc IRES is up to seven times more active than that of c-myc, suggesting that neuronal-specific non-canonical trans-acting factors are used by the N-myc but not the c-myc IRES. N-myc expression is increased by gene amplification in many neuroblastomas, but this is the first example of a translational mechanism by which N-myc expression could be further increased. The discovery of an IRES that displays enhanced activity in neuronal cell lines has important potential as a tool for protein expression in neural tissue.

c-Myc protein synthesis is initiated from the internal ribosome entry segment during apoptosis

Recent studies have shown that during apoptosis protein synthesis is inhibited and that this is in part due to the proteolytic cleavage of eukaryotic initiation factor 4G (eIF4G). Initiation of translation can occur either by a cap-dependent mechanism or by internal ribosome entry. The latter mechanism is dependent on a complex structural element located in the 5' untranslated region of the mRNA which is termed an internal ribosome entry segment (IRES). In general, IRES-mediated translation does not require eIF4E or full-length eIF4G. In order to investigate whether cap-dependent and cap-independent translation are reduced during apoptosis, we examined the expression of c-Myc during this process, since we have shown previously that the 5' untranslated region of the c-myc proto-oncogene contains an IRES. c-Myc expression was determined in HeLa cells during apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand. We have demonstrated that the c-Myc protein is still expressed when more than 90% of the cells are apoptotic. The presence of the protein in apoptotic cells does not result from either an increase in protein stability or an increase in expression of c-myc mRNA. Furthermore, we show that during apoptosis initiation of c-myc translation occurs by internal ribosome entry. We have investigated the signaling pathways that are involved in this response, and cotransfection with plasmids which harbor either wild-type or constitutively active MKK6, a specific immediate upstream activator of p38 mitogen-activated protein kinase (MAPK), increases IRES-mediated translation. In addition, the c-myc IRES is inhibited by SB203580, a specific inhibitor of p38 MAPK. Our data, therefore, strongly suggest that the initiation of translation via the c-myc IRES during apoptosis is mediated by the p38 MAPK pathway.

Analysis of the c-myc IRES; a potential role for cell-type specific trans-acting factors and the nuclear compartment

The 5' UTR of c -myc mRNA contains an internal ribo-some entry segment (IRES) and consequently, c -myc mRNAs can be translated by the alternative mechanism of internal ribosome entry. However, there is also some evidence suggesting that c -myc mRNA translation can occur via the conventional cap-dependent scanning mechanism. Using both bicistronic and monocistronic mRNAs containing the c- myc 5' UTR, we demonstrate that both mechanisms can contribute to c- myc protein synthesis. A wide range of cell types are capable of initiating translation of c- myc by internal ribosome entry, albeit with different efficiencies. Moreover, our data suggest that the spectrum of efficiencies observed in these cell types is likely to be due to variation in the cellular concentration of non-canonical translation factors. Interestingly, the c -myc IRES is 7-fold more active than the human rhinovirus 2 (HRV2) IRES and 5-fold more active than the encephalomyocarditis virus (EMCV) IRES. However, the protein requirements for the c -myc IRES must differ significantly from these viral IRESs, since an unidentified nuclear event appears to be a pre-requisite for efficient c -myc IRES-driven initiation.