Bone lesions in multiple myeloma--the OPG/RANK-ligand system.

Multiple myeloma has recently been found to induce considerable imbalance in the newly identified system of osteoprotegerin (OPG), receptor activator of nuclear factor KB ligand (RANKL) and RANK. The binding of RANKL to RANK on the surface of osteoclastic precursors in the presence of m-CSF activates the signalling pathways for differentiation and proliferation of an osteoclastic line. OPG is a decoy circulating receptor for RANKL which blocks its binding to RANK. There are at least three mechanisms by which myeloma cells affects the OPG/ RANKL/RANK system: 1: The adhesion between the myeloma / stromal cells and the osteoblastic precursors stimulates the system by increasing the production of RANKL. 2: Some myeloma lines produce independently membrane-bound or free RANKL. 3: The normal and mutated plasma cells bind, degrade and block the OPG production from the stromal cells. The OPG/RANKL/RANK system is the latest therapeutic target in the treatment of myeloma bone disease. The first results from the application of a synthetic analogue of OPG, as well as of RANKL antagonists or RANK inhibitors show decrease of the number of osteoclasts, osteolytic lesions and M-gradient.