Tumour-derived miRNAs and bone metastasis.

Skeletal metastases are complications of epithelial cancers, among which breast, prostate and lung carcinomas are the most osteotropic. In primary tumours, a subset of cancer cells undergoes epithelial-mesenchymal transition, acquires mobility to migrate into the surrounding stroma and seeds at distant sites to grow. The specific development of bone metastasis requires the recruitment of circulating tumour cells in the bone marrow, their adaptation to survive in the surrounding microenvironment where they alter the functions of osteoclasts and osteoblasts, and hijack signals coming from the bone matrix. Each of the molecular pathways underlining these steps is regulated by multiple factors, through the tight control of genes expressed by cancer cells interacting with cells from the bone microenvironment. In this context, miRNAs can act as master regulators of gene expression to control multiple aspects of bone metastasis formation, including cancer cell escape from the primary tumour site, cancer cell dissemination to bone and invasion of the bone marrow, as well as secondary outgrowth and tumour-stroma cell interactions. In the clinic, specific miRNA signatures have been identified in osteotropic cancer cells, raising the possibility that miRNAs could be used as biomarkers of bone metastasis. The regulatory activity of miRNAs in the bone microenvironment also suggests that miRNAs could be promising therapeutic targets.