MicroRNA-independent modulation of DICER1 expression by hAgo2

Many proteins, including DICER1 and hAgo2, are involved in the biogenesis of the microRNAs. Whether hAgo2 regulates DICER1 expression is unknown. Exogenously over-expressed hAgo2 suppressed DICER1 expression at the levels of both protein and mRNA, and the reduction in hAgo2 expression enhanced DICER1 expression. Precursor miRNA processing mediated by DICER1 was also modulated by hAgo2. However, hAgo2 protein did not suppress DICER1 promoter activity. Therefore, hAgo2 protein probably regulates DICER1 expression at the post-transcriptional level. Indeed, hAgo2 protein inhibited the reporter assay of the DICER1 mRNA 3'-UTR. Previous reports have demonstrated that miRNAs (e.g., let-7, miR-103/107) inhibited DICER1 expression post-transcriptionally. However, hAgo2 still suppressed DICER1 expression in the cells depleted of these miRNAs. Moreover, the reporter activities of DICER1 mRNA 3'-UTR without these miRNA binding sites were still suppressed by hAgo2. Therefore, in addition to a miRNA-dependent pathway, hAgo2 can also modulate DICER1 expression through a miRNA-independent mechanism. Downregulation of DICER1 expression was further proven to be dependent on both hAgo2 and AUF1 proteins. Interactions of hAgo2 and AUF1(s) proteins were demonstrated by the co-immunoprecipitation assay. As expected, hAgo2 could not suppress the DICER1 mRNA 3'-UTR reporter with a mutation in the potential AUF1-binding site. Thus, downregulation of DICER1 expression through the 3'-UTR requires both hAgo2 and AUF1.