[Cytokines, growth factors, and metabolic bone disease]

Cytokines are polypeptides that bind to membrane receptors and may act in an endocrine, paracrine or autocrine way. Several cytokines and growth factors may be produced by bone cells, stored in the matrix or act on them. Osteoclasts derive from the bone marrow stem cell and, as monocytes, belong to the family of tissue macrophages. Their specific function is bone resorption. Interleukin 1, 6 and 11, transforming growth factor and tumor necrosis factor stimulate osteoclast mediated bone resorption. Interleukin 1 is the most potent bone resorption agent and seems to be identical to osteoclast activation factor, identified in multiple myeloma. The role of interleukin 1, 6, 11 and tumor necrosis factors in postmenopausal osteoporosis triggered by the fall in estrogen levels, has not been well defined yet. Cytokines that increase bone formation are insulin like growth factors I and II, transforming growth factor, platelet derived growth factor and bone morphogenic proteins. Probably, tumor necrosis factor and interferon-gamma have a depressor effect on bone formation. Cytokines and growth factors, liberated from bone cells or from the matrix during osteoclastic work, could be the signals responsible for coupling bone formation and resorption.