The estrogen receptor-α-induced microRNA signature regulates itself and its transcriptional response

Following estrogenic activation, the estrogen receptor-alpha (ER alpha) directly regulates the transcription of target genes via DNA binding. MicroRNAs ( miRNAs) modulated by ER alpha have the potential to fine tune these regulatory systems and also provide an alternate mechanism that could impact on estrogen-dependent developmental and pathological systems. Through a microarray approach, we identify the subset of microRNAs ( miRNAs) modulated by ER alpha, which include upregulation of miRNAs derived from the processing of the paralogous primary transcripts (pri-) mir-17-92 and mir-106a-363. Characterization of the mir-17-92 locus confirms that the ER alpha target protein c-MYC binds its promoter in an estrogen-dependent manner. We observe that levels of pri-mir-17-92 increase earlier than the mature miRNAs derived from it, implicating precursor cleavage modulation after transcription. Pri-mir-17-92 is immediately cleaved by DROSHA to pre-miR-18a, indicating that its regulation occurs during the formation of the mature molecule from the precursor. The clinical implications of this novel regulatory system were confirmed by demonstrating that pre-miR-18a was significantly upregulated in ER alpha-positive compared to ER alpha-negative breast cancers. Mechanistically, miRNAs derived from these paralogous pri- miRNAs (miR-18a, miR-19b, and miR-20b) target and downregulate ER alpha, while a subset of pri-miRNA-derived miRNAs inhibit protein translation of the ER alpha transcriptional p160 coactivator, AIB1. Therefore, different subsets of miRNAs identified act as part of a negative autoregulatory feedback loop. We propose that ER alpha, c-MYC, and miRNA transcriptional programs invoke a sophisticated network of interactions able to provide the wide range of coordinated cellular responses to estrogen.