TRANSFERRIN PROMOTES ENDOTHELIAL-CELL MIGRATION AND INVASION - IMPLICATION IN CARTILAGE NEOVASCULARIZATION

During endochondral bone formation, avascular cartilage differentiates to hypertrophic cartilage that then undergoes erosion and vasculariza tion leading to bone deposition. Resting cartilage produces inhibitors of angiogenesis, shifting to production of angiogenic stimulators in hypertrophic cartilage. A major protein synthesized by hypertrophic ca rtilage both in vivo and in vitro is transferrin. Here we show that tr ansferrin is a major angiogenic molecule released by hypertrophic cart ilage, Endothelial cell migration and invasion is stimulated by transf errins from a number of different sources, including hypertrophic cart ilage. Checkerboard analysis demonstrates that transferrin is a chemot actic and chemokinetic molecule. Chondrocyte-conditioned media show si milar properties. Polyclonal anti-transferrin antibodies completely bl ock endothelial cell migration and invasion induced by purified transf errin and inhibit the activity produced by hypertrophic chondrocytes b y 50-70% as compared with controls, Function-blocking mAbs directed ag ainst the transferrin receptor similarly reduce the endothelial migrat ory response. Chondrocytes differentiating in the presence of serum pr oduce transferrin, whereas those that differentiate in the absence of serum do not, Conditioned media from differentiated chondrocytes not p roducing transferrin have only 30% of the endothelial cell migratory a ctivity of parallel cultures that synthesize transferrin, The angiogen ic activity of transferrins was confirmed by in vivo assays on chicken egg chorioallantoic membrane,showing promotion of neovascularization by transferrins purified from different sources including conditioned culture medium. Based on the above results, we suggest that transferri n is a major angiogenic molecule produced by hypertrophic chondrocytes during endochondral bone formation.