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, monocytes, and macrophages caused by the absence of functional macrophage-colony stimulating factor (M-CSF). It is also well known that the number of osteoclasts 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-l), the receptor for VEGF, and that recombinant human VEGF (rhVEGF) induced a significant number of osteoclasts in op/op mice. In this study, we confirmed that a single injection of 5 pg rhVEGF was sufficient to recruite osteoclasts in the femora of op/op mice. However, the number of osteoclasts recruited by rh\/EGF 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 Flt-1 was expressed in the osteoclasts induced by rhVEGF injection as well as in naturally appeared osteoclasts in op/op mice. These results indicate that the osteoclast precursor can differentiate to mature osteoclast at least three days after rh\/EGF injection and the stimulatory potency of rh\/EGF is lower than that of rhM-CSF. _