Simple SummaryBone metastatic prostate cancer is a lethal disease, and improved treatment strategies are urgently needed. MicroRNAs (miRNAs) are short non-coding RNAs with therapeutic possibilities in cancer. The current study compared levels of 1510 miRNAs in prostate cancer bone metastases with corresponding levels in the normal prostate and in localized prostate tumors, and found downregulation of a set of miRNAs during disease progression (miRNA-1, -23c, -143-3p, -143-5p, -145-3p, -205-5p, -221-3p, -222-3p and -4328). The downregulation of those miRNAs during the development of bone metastases may lead to a loss of inhibitory effects on prostate cancer cells. Functional studies confirmed the inhibiting effects of miRNA-23c and -4328 on prostate cancer cell growth in culture, while no clear effects of miRNA-23c were observed on tumor growth in mice.AbstractMicroRNAs (miRNAs) are aberrantly expressed in prostate cancer (PC), but comprehensive knowledge about their levels and function in metastatic PC is lacking. Here, we explored the differential expression of miRNA profiles during PC progression to bone metastasis, and further focused on the downregulation of miRNA-23c and -4328 and their impact on PC growth in experimental models. Using microarray screening, the levels of 1510 miRNAs were compared between bone metastases (n= 14), localized PC (n= 7) and benign prostate tissue (n= 7). Differentially expressed miRNAs (n= 4 increased andn= 75 decreased,p< 0.05) were identified, of which miRNA-1, -23c, -143-3p, -143-5p, -145-3p, -205-5p, -221-3p, -222-3p and -4328 showed consistent downregulation during disease progression (benign > localized PC > bone metastases). The downregulation of miRNA-23c and -4328 was confirmed by reverse transcription and quantitative polymerase chain reaction analysis of 67 metastasis, 12 localized PC and 12 benign prostate tissue samples. The stable overexpression of miRNA-23c and -4328 in the 22Rv1 and PC-3 cell lines resulted in reduced PC cell growth in vitro, and in the secretion of high levels of miRNA-23c (but not -4328) in extracellular vesicles. However, no tumor suppressive effects were observed from miRNA-23c overexpression in PC-3 cells subcutaneously grown in mice. In conclusion, bone metastases display a profound reduction of miRNA levels compared to localized PC and benign disease. The downregulation of those miRNAs, including miRNA-23c and -4328, may lead to a loss of tumor suppressive effects and provide biomarker and therapeutic possibilities that deserve to be further explored.