Vascular endothelial growth factor (VEGF) in cartilage neovascularization and chondrocyte differentiation: auto-paracrine role during endochondral bone formation

Vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) induces endothelial cell migration and proliferation in culture and is stro ngly angiogenic in vivo. VEGF synthesis has been shown to occur in both nor mal and transformed cells. The receptors for the factor have been shown to be localized mainly in endothelial cells, however, the presence of VEGF syn thesis and the VEGF receptor in cells other than endothelial cells has been demonstrated. Neoangiogenesis in cartilage growth plate plays a fundamenta l role in endochondral ossification. We have shown that, in an avian in vit ro system for chondrocyte differentiation, VEGF was produced and localized in cell clusters totally resembling in vivo cartilage. The factor was synth esized by hypertrophic chondrocytes and was released into their conditioned medium, which is highly chemotactic for endothelial cells. Antibodies agai nst VEGF inhibited endothelial cell migration induced by chondrocyte condit ioned media. Similarly, endothelial cell migration was inhibited also by an tibodies directed against the VEGF receptor 2/Flk1 (VEGFR2). In avian and m ammalian embryo long bones, immediately before vascular invasion, VEGF was distinctly localized in growth plate hypertrophic chondrocytes. In contrast , VEGF was not observed in quiescent and proliferating chondrocytes earlier in development. VEGF receptor 2 colocalized with the factor both in hypertrophic cartilage in vivo and hypertrophic cartilage engineered in vitro, suggesting an autoc rine loop in chondrocytes at the time of their maturation to hypertrophic c ells and of cartilage erosion. Regardless of cell exposure to exogenous VEG F, VEGFR-2 phosphorylation was recognized in cultured hypertrophic chondroc ytes, supporting the idea of an autocrine functional activation of signal t ransduction in this non-endothelial cell type as a consequence of the endog enous VEGF production. In summary we propose that VEGF is actively responsible for hypertrophic ca rtilage neovascularization through a paracrine release by chondrocytes, wit h invading endothelial cells as a target. Furthermore, VEGF receptor locali zation and signal transduction in chondrocytes strongly support the hypothe sis of a VEGF autocrine activity also in morphogenesis and differentiation of a mesoderm derived cell.