MicroRNAs (miRNAs) comprise a class of short, non-coding RNAs that regulate protein synthesis through posttranscriptional modifications. In this study, we found significant upregulation of miR-18a in prostate cancer specimens and prostate cancer cell lines compared with the normal controls. MiRNAs can be separated into two groups based on whether they regulate tumor suppressors or oncogenes. In our previous study, we found that miR-18a, which belongs to the miR17-92 cluster, is upregulated in prostate cancer; the objective of this study was to investigate the associated regulatory mechanisms. We found that miR-18a is upregulated in clinical tumor specimens and cancer cell lines. Our bioinformatics analysis showed that the serine/threonine-protein kinase 4 (STK4) 30 untranslated region contains a highly conserved binding site for the miR-18a seed region. Luciferase reporter assays were performed to indicate that STK4 is a direct target of miR-18a. Interestingly, miR-18a knockdown decreased cell growth in prostate cancer cells and significantly decreased prostate tumor growth in in vivo nude mice experiments through STK4- mediated dephosphorylation of AKT and thereby inducing apoptosis. Our results suggest that miR-18a acts as an oncomiR targeting STK4 in prostate cancer, and inhibition of miR-18a expression may offer therapeutically beneficial option for prostate cancer treatment.
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