Identification and Characterization of the Insulin-like Growth Factor I Receptor in Mature Rabbit Osteoclasts

In this study, the insulin-like growth factor I (IGF-I) receptor was identified in rabbit osteoclasts at mRNA and protein levels by in situ hybridization and autoradiography, respectively. Using highly purified mature osteoclasts, the IGF-I receptor was characterized on the molecular level according to its size and its affinity and number per osteoclast by isolation of the receptor-ligand complex and by binding studies, respectively, and on the cellular level according to the response of mature osteoclasts to IGF-I stimulation. In situ hybridization and autoradiography experiments showed that osteoclasts express IGF-I receptor mRNA and IGF-I binding sites. Chemical cross-linking of 125I-IGF-I bound to the purified mature osteoclasts and subsequent sodium dodecyl sulfide-polyacrylamide gel electrophoresis revealed the specific binding of 125I-IGF-I in complexes with molecular masses of 130 and 230-RD consistent with binding to the IGF-I receptor. In competition experiments, 125I-IGF-I binding to mature osteoclasts was dose-dependently reduced by unlabeled IGF-I in the picomolar range, whereas 20 nM insulin did not reduce the binding of 125I-IGF-I binding. The calculated receptor number was 6000 per osteoclast, and the Kd was 0.10 nM. Searching for a role of the IGF-I receptor in mature osteoclasts, we found no significant influence of IGF-I on the levels of the proform of matrix metaloproteinase 9 and tartrate-resistant acid phosphatase. However, the induction of nuclear fragmentation in serum-depleted cultures of purified mature osteoclasts was dose-dependently inhibited by IGF-I in the picomolar range, but not by 1 nM insulin. These data show that functionally active IGF-I receptor is present in mature osteoclasts.