The OPG/RANK/RANKL system and its interaction with TRAIL. RANKL binds to RANK on osteoprogenitor cells and controls osteoclastogenesis and bone resorption. OPG regulates negatively this interaction and therefore inhibits osteoclast development and activation. TRAIL is being secreted by monocytes and is responsible for their ability to induce the apoptotic mechanisms of tumor cells. OPG facilitates the survival of prostate cancer cells due to its ability to bind to and inhibit the TRAIL death-activating receptors.