RANK expression on breast cancer cells promotes skeletal metastasis

RANK ligand (RANKL), acting through its cognate receptor RANK, is a key factor for bone remodeling and metastasis by regulating the differentiation, survival and activation of osteoclasts. RANKL is also crucial for the development of mouse mammary glands during pregnancy and has been recently linked to the etiology of breast cancer via its direct activity on RANK-expressing normal or transformed breast epithelial cells, leading to increased mitogenesis, enhanced regenerative potential of mammary stem cells, and increased invasion and migration. We demonstrate that higher RANK expression in MDA-MB-231 breast cancer cells (MDA-231-RANK cells) is sufficient to confer a significantly greater metastatic growth rate in the bone compared with MDA-MB-231 cells which do not express high levels of RANK. Blockade of osteoclastic bone resorption, achieved with treatment by either RANKL inhibition or zoledronic acid, did reduce skeletal tumor progression of MDA-231-RANK cells suggesting that the vicious cycle contributes to metastatic growth. However, RANKL inhibition reduced skeletal growth of MDA-231-RANK tumors to a significantly greater extent than zoledronic acid, indicating that skeletal growth of RANK-positive tumors is also driven by direct RANKL effects. RANKL stimulated the expression of multiple genes associated with cell invasive behavior, including several matrix metalloproteinases and other genes previously defined as part of a bone metastasis gene signature. These data indicate that RANKL provokes breast cancer bone metastases via two distinct, but potentially overlapping mechanisms: stimulation of tumor-associated osteoclastogenesis and stimulation of RANK-expressing tumor cells.