Dose- and time-dependent changes in osteoclast induction after a single injection of vascular endothelial growth factor in osteopetrotic mice

Osteopetrotic (op/op) mice have a severe deficiency of osteoclasts, monocyt es, and macrophages caused by the absence of functional macrophage-colony s timulating factor (M-CSF). It is also well known that the number of osteocl asts in op/op mice exhibits a gradual increase naturally, demonstrating the self-healing of osteopetrosis. Our recent study showed that osteoclasts in normal mice expressed fms-like tyrosine kinase (Flt-1), the receptor for V EGF, and that recombinant human VEGF (rhVEGF) induced a significant number of osteoclasts in op/op mice. In this study, we confirmed that a single inj ection of 5 mu g rhVEGF was sufficient to recruite osteoclasts in the femor a of op/op mice. However, the number of osteoclasts recruited by rhVEGF was less than that by rhM-CSF. The number of osteoclasts reached to a maximum level three days after the injection of rhVEGF or rhM-CSF. We found that Fl t-1 was expressed in the osteoclasts induced by rhVEGF injection as well as in naturally appeared osteoclasts in op/op mice. These results indicate th at the osteoclast precursor can differentiate to mature osteoclast at least three days after rhVEGF injection and the stimulatory potency of rhVEGF is lower than that of rhM-CSF.