Mir-17-5p regulates breast cancer cell proliferation by inhibiting translation of AIB1 mRNA

MicroRNAs are an extensive family of 22-nucleotide-long noncoding RNAs expressed in a wide range of eukaryotes, including humans, and they are important in development and disease. We found that microRNA Mir-17-5p has extensive complementarity to the mRNA of AIB1 (named for “amplified in breast cancer 1”). Cell culture experiments showed that AIB1 expression was downregulated by Mir-17-5p, primarily through trans-lational inhibition. Expression of Mir-17-5p was low in breast cancer cell lines. We also found that downregu-lation of AIB1 by Mir-17-5p resulted in decreased estrogen receptor-mediated, as well as estrogen receptor-independent, gene expression and decreased proliferation of breast cancer cells. Mir-17-5p also completely abrogated the insulin-like growth factor 1-mediated, anchorage-independent growth of breast cancer cells. Our results reveal that Mir-17-5p has a role as a tumor suppressor in breast cancer cells. MicroRNAs (miRNAs) are genomically encoded, 22-nu-cleotide-long noncoding RNAs found in many organisms. miRNAs are produced from primary RNA polymerase II tran-scripts by sequential processing in the nucleus and cytoplasm (26, 27). Nuclear precursor RNAs are cleaved by the endo-nuclease Drosha in a “microprocessor complex ” to releas