DOWN-REGULATION OF COLONY-STIMULATING FACTOR-I (CSF-1) BINDING BY CSF-1 IN ISOLATED OSTEOCLASTS

Colony-stimulating factor-1 (CSF-1), also called macrophage colony-sti mulating factor, is the growth factor for the cells of the mononuclear phagocytic system. Furthermore, CSF-1 is essential in osteoclastogene sis and also affects mature osteoclasts. The receptor for CSF-1 was de monstrated on cells of the osteoclast lineage, with highest levels on the mature cells. This study investigated whether the binding of CSF-1 to isolated rat osteoclasts is modulated by the growth factor itself. Exposure of osteoclasts to CSF-1 for 1 hour virtually abolished bindi ng of the growth factor. After removal of CSF-1, binding sites were re stored within 4 hours. This recovery was blocked by cycloheximide, ind icating the dependence on new protein synthesis for reexpression of re ceptors on the cell surface. The observed downregulation of CSF-1 bind ing sites might be a mechanism to control the effects of the growth fa ctor on mature osteoclasts.