Macrophage colony-stimulating factor restores bone resorption in op/op bone in vitro in conjunction with parathyroid hormone or 1,25-dihydroxyvitamin D3.

The in vivo administration of macrophage colony-stimulating factor (M-CSF) restores osteoclastogenesis and bone resorption in the op/op murine osteopetrosis. In vitro, exogenous M-CSF has been shown to be necessary for the generation of osteoclast-like cells in cocultures of hematopoietic and mesenchymal cells obtained from this mutant. In this study we investigated the capacity of M-CSF and other cytokines and hormones, alone or in combination, to induce bone resorption in explants of op/op metatarsals and metacarpals prelabeled with 45Ca. The effect on bone resorption was verified by counting the number of osteoclasts generated in the mineralized matrix. No osteoclast formation and no bone resorption were observed in the absence of M-CSF. M-CSF alone had only a slight effect at the high concentration of 10(4) units/ml. Addition of PTH or 1,25-(OH)2D3 together with M-CSF induced both osteoclastogenesis and bone resorption. The release of 45Ca was linear with time up to 15 days. PTH or 1,25-(OH)2D3 could not be substituted by TNF-alpha or IL-1, whereas IL-6 had a weak effect. M-CSF could not be replaced by GM-CSF. This study further emphasizes the role of M-CSF, PTH, and 1,25-(OH)2D3 in osteoclastogenesis.

Effect of macrophage colony-stimulating factor on in vitro osteoclast generation and bone resorption.

To investigate the role of recombinant human macrophage colony-stimulating factor (rhM-CSF) on in vitro bone resorption, two bone explants, each at a different developmental stage, were adopted, namely 1) radii and 2) metatarsals of 17-day-old embryonic mice. At this stage of gestation, bone resorption in the radii is exclusively dependent upon fusion of osteoclast precursors and activation of mature osteoclasts, whereas in metatarsals it is dependent upon the generation of new osteoclasts. rhM-CSF showed no effect in radii, but stimulated 45Ca release in metatarsals, when they were either intact or periosteum stripped in coculture with embryonal liver as a source of hemopoietic progenitors. The rhM-CSF-induced increase in 45Ca release was paralleled by a higher number of osteoclasts. The stimulating effect was found to be in a concentration range between 250-500 U/ml M-CSF. The action of rhM-CSF was blocked by irradiation, indicating that it is dependent upon cell proliferation. These results, thus, show that M-CSF stimulates bone resorption only when it is dependent on generation of new osteoclasts. M-CSF does not appear to have any effect on the activity of mature osteoclasts. The mechanism of action might be direct on osteoclast precursors or indirect on accessory cells influencing osteoclast generation.