Retinoic acid-induced heparin binding protein (RIHB) binds to embryonal chondrocytes and cartilage primarily via proteoglycans.

Retinoic acid-induced heparin binding protein (RIHB) is a highly basic, secreted polypeptide expressed during early chick embryogenesis. We have characterized the binding of 125I-labeled RIHB to embryonal chondrocytes in culture. No saturable, high-affinity binding can be observed on these cells. Furthermore, no 125I-labeled RIHB was internalized into the chondrocytes at 37 degrees C. The low-affinity binding of 125I-labeled RIHB observed can be competed with another heparin binding factor, fibroblast growth factor 2, as efficiently as with unlabeled RIHB. The binding can also be almost completely inhibited by preincubation of the 125I-labeled RIHB with heparin or with a monoclonal antibody which recognizes the heparin binding site of both RIHB and HBNF. When cross-linking experiments are performed with 125I-labeled RIHB, specific RIHB-containing high-molecular-weight complexes are observed; however, these represent only a very small fraction of the bound material. Immunohistochemical analyses of embryonic wing cartilage demonstrate that a significant fraction of bound RIHB can be removed from unfixed tissue simply by rinsing with phosphate-buffered saline. The remaining RIHB can be removed partially by incubation with heparitinase I or III and completely when the incubation is performed with chondoitinase A, B, C. These results demonstrate that RIHB binds to embryonal chondrocytes and cartilage primarily through proteoglycans of both heparan sulfate and chondroitin sulfate types.