Stimulation of Hepatitis C Virus RNA translation by microRNA-122 occurs under different conditions in vivo and in vitro

Translation of the Hepatitis C Virus (HCV) positive strand RNA genome is directed by an internal ribosome entry site (IRES) in the viral RNA's 5'-untranslated region (5'-UTR). HCV propagates preferentially in the liver, and HCV translation is stimulated by the liver-specific microRNA-122 (miR-122) acting on two target sites in the 5'-UTR. This stimulation is effective in living cells containing miR-122 and also in the rabbit reticulocyte lysate in vitro-translation system after addition of miR-122. Another RNA sequence located in the Core protein coding sequence can base-pair in a long-range RNA-RNA interaction to the HCV 5'-UTR, overlapping with the miR-122 target sites and the short spacer between them, and thereby inhibits HCV translation. Here we show genetic evidence that in reticulocyte lysate single-stranded miR-122 interferes with this inhibitory long-range RNA-RNA interaction and thereby contributes to enhanced HCV translation, involving not only the 5'-seed sequence of miR-122 but also sequences at its 3'-end. Also RNA oligonucleotides shorter than a typical microRNA stimulate HCV translation, confirming that in the reticulocyte lysate the stimulation of HCV translation functions by displacement of the inhibitory long-range interaction by miR-122. In contrast, in transfected HuH-7 hepatoma cells and in HeLa cells this interference of miR-122 with the inhibitory long-range RNA-RNA interaction plays not a major role, but only duplex miR-122 RNAs of the correct length stimulate HCV translation. These results suggest that: (1) the processing of the microRNA precursors and (2) the events occurring at the HCV RNA differ between cells and reticulocyte lysate.