Hypertrophic chondrocytes in the epiphyseal growth plate express the angiogenic protein vascular endothelial growth factor (VEGF). To determine the role of VEGF in endochondral bone formation, we inactivated this factor through the systemic administration of a soluble receptor chimeric protein (Flt-(1-3)-IgG) to 24-day-old mice. Blood vessel invasion was almost completely suppressed, concomitant with impaired trabecular bone formation and expansion of hypertrophic chondrocyte zone. Recruitment and/or differentiation of chondroclasts, which express gelatinase B/matrix metalloproteinase-9, and resorption of terminal chondrocytes decreased. Although proliferation, differentiation and maturation of chondrocytes were apparently normal, resorption was inhibited. Cessation of the anti-VEGF treatment was followed by capillary invasion, restoration of bone growth, resorption of the hypertrophic cartilage and normalization of the growth plate architecture. These findings indicate that VEGF-mediated capillary invasion is an essential signal that regulates growth plate morphogenesis and triggers cartilage remodeling. Thus, VEGF is an essential coordinator of chondrocyte death, chondroclast function, extracellular matrix remodeling, angiogenesis and bone formation in the growth plate.
Vascular endothelial growth factor (VEGF) is an essential regulator of normal and abnormal blood vessel growth. A monoclonal antibody (mAb) that targets VEGF suppresses tumor growth in murine cancer models and human patients. We investigated cellular and molecular events that mediate refractoriness of tumors to anti-angiogenic therapy. Inherent anti-VEGF refractoriness is associated with infiltration of the tumor tissue by CD11b+Gr1+ myeloid cells. Recruitment of these myeloid cells is also sufficient to confer refractoriness. Combining anti-VEGF treatment with a mAb that targets myeloid cells inhibits growth of refractory tumors more effectively than anti-VEGF alone. Gene expression analysis in CD11b+Gr1+ cells isolated from the bone marrow of mice bearing refractory tumors reveals higher expression of a distinct set of genes known to be implicated in active mobilization and recruitment of myeloid cells. These findings indicate that, in our models, refractoriness to anti-VEGF treatment is determined by the ability of tumors to prime and recruit CD11b+Gr1+ cells.
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