Short hairpin type of dsRNAs that are controlled by tRNAVal promoter significantly induce RNAi-mediated gene silencing in the cytoplasm of human cells

The post-transcriptional gene silencing in animals and plants is called RNA interference (RNAi). Guides for the sequence-specific degradation of mRNA are 21-nt small interfering RNAs (siRNAs) that are generated by Dicer-dependent cleavage from longer double-stranded RNAs (dsRNAs). To examine the relationship between the localization of dsRNA and the target cleavage of RNAi in human cells, we constructed five kinds of dsRNA expression vector that were controlled by tRNA(Val) or U6 promoter. Transcripts of tRNA-dsRNA were consistently localized in the cytoplasm and were efficiently processed by Dicer. In contrast, transcripts of tRNA-dsRNA were not processed in cells that expressed Dicer-directed ribozymes. In addition, transcripts of U6-dsRNA were basically localized in the nucleus and were not significantly processed, unless the transcripts of U6-dsRNAs possessed a microRNA-based loop motif: in the latter case, U6-dsRNAs with a microRNA-based loop were transported to the cytoplasm and were effectively processed. Moreover, tRNA-dsRNA directed against a mutant k-ras transcript cleaved its target mRNA efficiently in assays of RNAi not only in vitro with a cytoplasmic extract but also in vivo. Therefore, it appears that RNAi in human cells occur in the cytoplasm. Importantly, the same tRNA-dsRNA did not affect the degradation of the normal k-ras mRNA in vitro and in vivo. Our tRNA-dsRNA technology should be a powerful tool for studies of the mechanism of RNAi and the functions of various genes in mammalian cells with potential utility as a therapeutic agent.