Osteoclasts Are Important for Bone Angiogenesis

Osteoclastogenesis and angiogenesis are correlated in bone during physiological and pathological processes including development, fracture healing, bone metastases and inflammatory bone disease. However, it is unclear if and how these processes are linked. This dissertation investigates a possible causative role for osteoclasts in bone angiogenesis. First, changes in osteoclast formation and activity affected angiogenesis in a parallel fashion. Osteoclast inhibition decreased angiogenesis, while osteoclast stimulation increased angiogenesis in fetal mouse metatarsal explants. Likewise, osteoclast stimulation also increased angiogenesis in mouse calvaria in vivo, thus showing that osteoclasts and angiogenesis are linked. Further studies were conducted to determine the mechanism by which osteoclasts may increase angiogenesis. Angiogenic factor expression by osteoclasts was analyzed by reverse-trancriptase PCR and Q-PCR angiogenesis arrays of human bone marrow osteoclasts. MMP-9 was the most highly expressed osteoclast angiogenic factor at the mRNA level. Because MMP-9 is important for osteoclast and blood vessel invasion of the growth plate and fracture calluses, the role of MMP-9 in osteoclast stimulated angiogenesis was studied in depth. Osteoclast stimulation with RANKL or PTHrP failed to stimulate angiogenesis in MMP-9-/- mouse calvaria or metatarsal explants. Surprisingly, osteoclast stimulation was dramatically blunted in MMP-9-/- calvaria or metatarsal explants. However, the number of vessels per osteoclast was not different between WT and MMP-9-/- mice, indicating that osteoclasts lacking MMP-9 do not have an intrinsic angiogenic defect. Further, bone marrow cultures from WT and MMP-9-/- mice formed similar numbers of osteoclasts, demonstrating that osteoclast differentiation or precursor number is not responsible for the inability of PTHrP or RANKL to increase osteoclastogenesis in MMP-9-/- mice. These results suggest that MMP-9 is important for osteoclast-stimulated angiogenesis by affecting the number of osteoclasts at the angiogenic site due to its previously reported effects on osteoclast migration. These studies greatly increase our understanding of angiogenesis in bone and suggest an important role for osteoclasts in angiogenesis during bone development, fracture healing, bone metastasis, inflammatory bone diseases and the potential effects of osteoclast inhibitory agents on angiogenesis