Intra-articular Injection of HB-IGF-1 Sustains Delivery of IGF-1 to Cartilage through Binding to Chondroitin Sulfate

Objective—IGF-1 stimulates cartilage repair but is not a practical therapy due to its short halflife. 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. Here, we first examined the mechanism by which HB-IGF-1 is retained in cartilage. We then tested 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 requirement of heparan sulfate (HS) or chondroitin sulfate (CS) glycosaminoglycans 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 glycosaminoglycans were examined by surface plasmon resonance and ELISA. 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 intra-articular injection in rat knees, HB-IGF-1 was retained in articular and meniscal cartilages, but not in tendon, consistent with enhanced delivery to CS-rich cartilage. Finally, HB-IGF-1 but not IGF-1 was retained in human cartilage explants. Conclusions—After intra-articular 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. Insulin-like growth factor-I (IGF-1) is known to be an important anabolic factor in cartilage homeostasis (1). IGF-1 not only promotes synthesis of aggrecan, link protein, and hyaluronan (2–4), it also inhibits proteoglycan degradation (5–7). IGF-1 is primarily produced by the liver and reaches cartilage through the synovial fluid (8–10), acting on chondrocytes through both autocrine and paracrine mechanisms (11,12). In multiple animal models of cartilage injury, chondrocytes transfected to overexpress IGF-1 have been successfully used to enhance cartilage repair (13,14). While IGF-1 may therefore be a potential therapeutic for cartilage repair, a clinically useful technique for acellular IGF-1 delivery to cartilage has yet to be developed. A successful Corresponding Author: Parth Patwari, Partners Research Building, 65 Landsdowne St. Rm. 283, Cambridge, MA 02139, Phone: 617-768-8246; Fax: 617-768-8270, [email protected]. Disclosure: RTL is listed as the inventor of a patent related to HB-IGF. NIH Public Access