Osteoprotegerin (OPG) produced by bone marrow stromal cells protects breast cancer cells from TRAIL-induced apoptosis

Advanced breast cancer is often associated with metastatic bone disease, causing a number of serious complications for the patients such as hypercalceamia, pain, nerve compression and fractures. The formation of bone metastases depends on complex interactions between tumour cells and the cells of the bone microenvironment, but the precise molecular mechanisms involved in the development of tumour-induced bone disease have not been identified. We have investigated the ability of bone marrow stromal cells (BMSC) isolated from breast cancer patients to generate osteoprotegerin (OPG), a molecule involved both in bone turnover and cell survival. The potential survival effects of OPG are mediated through binding to a member of the TNF super family, TNF-related Apoptosis Inducing Ligand (TRAIL), preventing association between TRAIL and its death-inducing receptors present on a number of tumour cell types. In the present report we show that bone marrow stromal cells isolated from breast cancer patients produce OPG when grown in culture. The levels of OPG present in BMSC conditioned medium is sufficient to protect breast cancer cells from undergoing TRAIL induced apoptosis. Our data suggest that bone-derived OPG may increase survival of breast cancer cells that reach the bone microenvironment as part of the metastatic process.