The functional significance of microRNA-145 in prostate cancer

Micro RNAs (miRNAs) are small noncoding RNAs that regulate the expression of protein-coding genes (He and Hannon, 2004; Lim et al, 2005; Volinia et al, 2006). MicroRNAs have important roles in numerous cellular processes, including development, proliferation and apoptosis (Carrington and Ambros, 2003). Characteristic miRNA signatures for several epithelial cancers, including breast, lung, pancreatic and gastric cancers, have been reported (Iorio et al, 2005; Yanaihara et al, 2006; Bloomston et al, 2007; Petrocca et al, 2008). Micro RNAs control gene expression by binding to complementary sites in the 3′ untranslated regions (3′ UTRs) of target mRNAs, triggering either translational inhibition or mRNA degradation (Baek et al, 2008). However, miRNAs can also function to positively regulate gene expression by binding to complementary or partial complementary sequences in the promoter regions of genes (Place et al, 2008). Recent studies have shown the aberrant expression of miRNAs in prostate cancer tissues and cell lines (Galardi et al, 2007; Porkka et al, 2007; Ozen et al, 2008). One of the miRNAs reported to be consistently downregulated in prostate tumours is miR-145, and it has been shown to act as a tumour suppressor in breast and colon cancers (Schepeler et al, 2008; Spizzo et al, 2009). In this study we analysed the functional significance of miR-145 in prostate cancer. We examined the expression level of miR-145 in 27 matched human prostate cancer laser microdissected tissue samples, and found a significant difference between adjacent normal and cancerous regions. We also investigated whether the mechanism of inactivation of miR-145 in prostate cancer cell lines is through methylation of its promoter. After combination treatments with low doses of genistein (G), 5-aza-2′-deoxycytidine (5-aza) and trichostatin A (TSA), there was an increase in the expression of miR-145, suggesting that silencing of miR-145 is through DNA methylation. To identify potential miR-145 targets, we performed microarray analyses in miR-145-overexpressing prostate cancer cells and found upregulation of the pro-apoptotic gene TNFSF10. We also observed that overexpression of miR-145 in PC3 cells led to cell cycle arrest and increased apoptosis. This is the first report on the mechanisms of inactivation of miR-145 through DNA methylation in prostate cancer.