Bone Abstracts

Prostate cancer (PCa) is the most common cancer in males. When patients develop metastasis, no curative therapy is available. To find new therapeutic options, it is crucial to understand how PCa cells induce metastasis. Recently, it was shown that PCa cells secrete small extracellular vesicles (EVs) that can be found in the circulation and in bones. Uptake of EVs by other cells may change their behaviour. We previously identified three miRNAs that were uniquely upregulated in metastatic PCa patients, though their contribution to metastasis remains to be studied. Therefore, we transduced PC-3, DU145 and RWPE-1 cells with three microRNAs resulting in overexpression. Effects on osteoclasts were determined by osteoclastogenesis and activity assays (coomassie staining of resorption pits) on bone chips using isolated human peripheral blood mononucleated cells. miRNA expression was determined by qRT-PCR, and PCa cell malignancy by the transwell invasion assay. PCa-cell invasion was stimulated by overexpression of two of the three microRNAs. Interestingly, in prostatic non-cancer RWPE-1-cells, a higher expression of the third microRNA increased the invasive potential, indicating that microRNAs have different functions at different progression stages. Importantly, the cellular miRNA levels correlated to the EV-miRNA levels. To determine whether the content of exosomes is actively used by osteoclasts, we used a Cre-lox system. THP1-monocytes were transduced with a reporter-construct, and two PCa cell lines with Cre, leading to active secretion of Cre within the PCa-EVs. When the EV-content is functionally used by the recipient THP1-cells, Cre recombines the reporter-gene resulting in a colour-switch. Indeed we observed a functional uptake in >50% of THP1-reporter⁺ cells, indicating that EVs can affect osteoclasts. Furthermore, osteoclastogenesis was increased when PCa-EVs were added. In conclusion, we identified miRNAs that enhance the malignant behavior of PCa-cells, and may also prepare the bone metastatic niche by activating osteoclasts at a distance through EVs.