Identification of hypoxia-inducible factor-1α as a novel target for miR-17-92 microRNA cluster

MicroRNAs (miRNAs) are a distinct class of small noncoding RNAs that posttranscriptionally repress expression of target genes through imperfect base pairing with the 3' untranslated region. We previously reported amplification and overexpression of the miR-17-92 miRNA cluster at 13q31.3 in lung cancers, as well as growth inhibition by treatment with antisense oligonucleotides against miR-17-5p and miR-20a, constituents of miR-17-92, specifically in miR-17-92-overexpressing lung cancer cell lines. Although these findings clearly suggested important roles of miR-17-92 overexpression in lung cancers, only a few targets for the miR-17-92 cluster have been identified thus far. In this study, we identified hypoxia-inducible factor (HIF)-1 alpha as a novel direct target for miR-17-92 through global expression profiling by mass spectrometric analysis using an isobaric tagging reagent, iTRAQ, combined with bioinformatic target prediction. This is the first report to describe negative regulation of HIF-1 alpha, by miRNA, which seemed to occur without disrupting the induction of HIF-1 alpha for cellular adaptation to hypoxia. In addition, overexpression of c-myc led to down-regulation of HIF-1 alpha and induction of miR-17-92, the latter of which was previously reported to be a transcriptional activation activity, suggesting that the induction of miR-17-92 may play a role at least in part in c-myc-mediated repression of HIF-1 alpha. Together with previous reports on the functional negative regulation of c-myc by HIF-1 alpha, our findings suggest the possible existence of an intricate and finely tuned circuit involving c-myc, miR-17-92, and HIF-1 alpha that may play a role in cancer cell proliferation under normoxia in a cellular context-dependent manner.