Non-equivalent Roles of AGO1 and AGO2 in mRNA Turnover and Translation of Cyclin D1 mRNA

Mammalian Argonaute proteins (AGO1-4), in combination with microRNAs (miRs), bind to target mRNAs to initiate degradation and/or translation repression, but the relationships between these two effects is unclear. Although the AGO isoforms of Drosophila and plants perform different functions, mammalian AGO isoforms are considered to be functionally degenerate in terms of miR loading and downstream silencing effects. However, we found that, in quiescent (G0) rat myoblasts transiting to the G1 phase, cyclin D1 (Ccnd1) mRNA was associated with two functionally distinct AGO-miR complexes. While AGO1-miR-1 down-regulated the mRNA level, AGO2-let-7 delayed the timing of translation. Knockdown (KD) of AGO2, or antisense-mediated depletion of Let-7, caused Ccnd1 translation to occur earlier, but had no significant effect on mRNA abundance. Conversely, down-regulation of either AGO1 or miR-1, resulted in elevated Ccnd1 mRNA levels at early times, but failed to affect the timing of translation. These results show that the two miR-mediated silencing effects, viz. mRNA decay and translation repression, are independent processes induced by individual AGO isoforms in association with specific miRs.