| Abstract: | Objective Insulin‐like growth factor 1 (IGF‐1) stimulates cartilage repair but is not a practical therapy due to its short half‐life. We have previously modified IGF‐1 by adding a heparin‐binding domain and have shown that this fusion protein (HB‐IGF‐1) stimulates sustained proteoglycan synthesis in cartilage. This study was undertaken to examine the mechanism by which HB‐IGF‐1 is retained in cartilage and to test whether HB‐IGF‐1 provides sustained growth factor delivery to cartilage in vivo and to human cartilage explants. Methods Retention of HB‐IGF‐1 and IGF‐1 was analyzed by Western blotting. The necessity of heparan sulfate (HS) or chondroitin sulfate (CS) glycosaminoglycans (GAGs) for binding was tested using enzymatic removal and cells with genetic deficiency of HS. Binding affinities of HB‐IGF‐1 and IGF‐1 proteins for isolated GAGs were examined by surface plasmon resonance and enzyme‐linked immunosorbent assay. Results In cartilage explants, chondroitinase treatment decreased binding of HB‐IGF‐1, whereas heparitinase had no effect. Furthermore, HS was not necessary for HB‐IGF‐1 retention on cell monolayers. Binding assays showed that HB‐IGF‐1 bound both CS and HS, whereas IGF‐1 did not bind either. After intraarticular injection in rat knees, HB‐IGF‐1 was retained in articular and meniscal cartilage, but not in tendon, consistent with enhanced delivery to CS‐rich cartilage. Finally, HB‐IGF‐1 was retained in human cartilage explants but IGF‐1 was not. Conclusion Our findings indicate that after intraarticular injection in rats, HB‐IGF‐1 is specifically retained in cartilage through its high abundance of CS. Modification of growth factors with heparin‐binding domains may be a new strategy for sustained and specific local delivery to cartilage. |
