Bone Abstracts

Signaling pathways crucial in bone development, including Wnt, are also upregulated in breast cancer cells to promote tumor growth in the skeleton, a process known as osteomimicry. Thus, we hypothesized that bone metastatic tumor cells also aberrantly express osteogenic miRNAs to support osteomimetic properties. We have previously shown that miR-218 is highly expressed in osteoblasts and promotes osteogenic differentiation. Interestingly, expression analysis revealed a significant up-regulation of miR-218 in bone metastatic MDA-MB-231 breast cancer cells compared to normal mammary epithelial cells. Furthermore, miR-218 was highly expressed in bone metastases biopsies from breast cancer patients, suggesting a positive role in bone metastasis. Indeed, delivery of miR-218 in MDA-MB-231-Luc cells promoted tumor growth in the bone microenvironment in vivo whereas inhibition of miR-218 impaired tumor growth. Signaling pathway analyses revealed a positive correlation between aberrant miR-218 expression and activation of Wnt signaling, demonstrated by reporter assays and expression of Wnt transcriptional mediators. Mechanistically, miR-218 targeted Wnt inhibitors Sclerostin and sFRP-2 and thus, delivery of miR-218 further enhanced while inhibition of miR-218 decreased Wnt activity. Importantly, the miR-218-induced expression of metastasis-related genes including bone sialoprotein, osteopontin and CXCR-4 was abolished by inhibition of Wnt signaling, indicating a Wnt-dependent regulation. Furthermore, PTHrP, a key cytokine promoting cancer-induced osteolysis was up-regulated in breast cancer cells by miR-218 in a Wnt-dependent manner. Consequently, conditioned medium from miR-218 expressing breast cancer cells increased Rankl in osteoblasts and supported osteoclast differentiation in osteoblast-osteoclast co-cultures. Importantly, antagonizing miR-218 reduced the expression of PTHrP and Rankl, inhibited osteoclast differentiation in vitro and in vivo, and prevented the development of osteolytic lesions in a preclinical metastasis model. In conclusion, we propose that miR-218 activates Wnt signaling to enhance metastatic properties of breast cancer cells and cancer-induced osteolytic disease. Therefore, antagonizing miR-218 represents a novel therapeutic intervention to prevent disease progression.