miR-122, a Mammalian Liver-Specific microRNA, is Processed from hcr mRNA and MayDownregulate the High Affinity Cationic Amino Acid Transporter CAT-1

These studies show that miR-122, a 22-nucleotide microRNA, is derived from a liver-specificnon-coding polyadenylated RNA transcribed from the gene hcr. The exact sequence of miR-122as well as the adjacent secondary structure within the hcr mRNA are conserved from mammalianspecies back to fish. Levels of miR-122 in the mouse liver increase to half maximal valuesaround day 17 of embryogenesis, and reach near maximal levels of 50,000 copies per averagecell before birth. Lewis et al (2003) predicted the cationic amino acid transporter (CAT-1 orSLC7A1) as a miR-122 target. CAT-1 protein and its mRNA are expressed in all mammaliantissues but with lower levels in adult liver. Furthermore, during mouse liver development CAT-1mRNA decreases in an almost inverse correlation with miR-122. Eight potential miR-122 targetsites were predicted within the human CAT-1 mRNA, with six in the 3’-untranslated region.Using a reporter construct it was found that just three of the predicted sites, linked in a 400-nucleotide sequence from human CAT-1, acted with synergy and were sufficient to stronglyinhibit protein synthesis and reduce mRNA levels. In summary, these studies followed theaccumulation during development of miR-122 from its mRNA precursor, hcr, through toidentification of what may be a specific mRNA target, CAT-1. Link to supplemental material:http://www.landesbioscience.com/supplement/changRNA1-2-sup.pdf