Differential activity of granulocyte-macrophage and macrophage colony stimulating factors on bone resorption in fetal rat long bone organ cultures.

In this study, the ability of recombinant human macrophage (M) and murine granulocyte-macrophage (GM) colony stimulating factor (CSF) to affect both basal and stimulated bone resorption in fetal rat long-bone organ cultures was assessed. It was found that M-CSF does not affect basal bone resorption or bone resorption stimulated by parathyroid hormone, recombinant human interleukin 1 beta, prostaglandin E2 (PGE2), and 1,25 dihydroxy vitamin D3. Specifically, M-CSF at concentrations as high as 30 nM (1 microgram/mL) did not modulate 45Ca release from fetal rat long bones stimulated by these agents. The addition of recombinant murine GM-CSF (at equal molar concentration to M-CSF) also did not affect bone resorption stimulated by parathyroid hormone and interleukin 1 beta. On the other hand, GM-CSF stimulated basal bone resorption over a 120-h period and augmented the resorption mediated by exogenous PGE2 over a 48-h incubation. In addition, GM-CSF was shown to stimulate production of endogenous PGE2 in cultures of bone rudiments. These effects on bone resorption were blocked by the addition of prostaglandin synthesis inhibitors and specific antibodies to murine GM-CSF. These data indicate that M-CSF does not act as a regulator of bone turnover, but GM-CSF may cause bone resorption by stimulating the synthesis of PGE2 in bone.