[Bone metastasis and RANKL]

The mice with a disruption of Rank or Rankl exhibit normal mammary development during puberty, but their mammary epithelium fails to proliferate and form lobuloalveolar structures during pregnancy, resulting in the death of newborns. Hormone replacement therapy is associated with an increased risk of breast cancer. Importantly, specific deletion of RANK in mammary epithelium cells prevents both the onset and progression of medroxyprogesterone acetate (MPA) -driven mammary cancer and impairs self-renewal of breast cancer stem cells. Furthermore, RANK is highly expressed in several cancer cells. Functionally, it has been shown that RANKL can stimulate the directed migration of mammary epithelial cells as well as prostate cancer and melanoma cells toward a source of RANKL. In an in vivo metastasis model, OPG reduced the tumor burden in bones and ameliorated clinical paralysis, but did not affect the frequency of the spread of metastases into other tissues. These findings show that the RANK/RANKL system is crucial for mammary development, breast tumorigenesis and bone metastasis.