Human Osteoarthritic Chondrocytes Possess an Increased Number of Insulin‐Like Growth Factor 1 Binding Sites but are Unresponsive to its Stimulation

Abstract

Objective. To characterize the insulin‐like growth factor 1 (IGF‐1) receptor in human osteoarthritic (OA) and normal adult chondrocytes. The biologic response of chondrocytes to IGF‐1 stimulation was examined, as was the presence and synthesis of IGF binding proteins (IGFBP) in these cells.Methods. Binding studies, Northern blot, immunohistochemical analysis, and affinity cross‐linking experiments were performed for characterization of the IGF receptor, and the latter method was also used for IGFBP determination. The biologic response was estimated via the incorporation of radiolabeled proline into a newly synthesized protein.Results. Binding experiments revealed a single class of binding sites. The mean ± SEM affinity (K_d) of normal chondrocytes was 1.4 ± 0.4 nM, with 26.8 ± 5.5 x 10³ binding sites/cell. OA chondrocytes had a lower affinity (K_d 15.4 ± 4.7 nM) and a higher density (1,178.3 ± 299.5 x 10³ binding sites/cell) compared with normal cells (P < 0.004 and P < 0.001, respectively). Immunohistochemical studies with a monoclonal antibody (MAb) against the type 1 IGF receptor (αIR3) showed increased staining in OA cartilage compared with normal tissue. Biologic responses of chondrocytes after IGF‐1 stimulation revealed that OA chondrocytes were unresponsive, whereas a 2.5‐fold increase in new protein synthesis was observed in normal cells. Competition studies in normal chondrocytes revealed that both IGF‐1 and IGF‐2 displaced radiolabeled IGF‐1 in a comparable manner; however, insulin at high concentration weakly competes. Moreover, MAb αIR3 effectively blocked specific binding in normal chondrocytes (77%), but not in OA chondrocytes (26%). Northern blot and covalent cross‐linking analyses revealed the specific band characteristic of type 1 receptor. With the latter technique, other bands corresponding to the IGFBPs were also detected. Comparison between normal and OA chondrocytes showed increased intensity of the IGFBP bands, particularly those corresponding to the IGFBP‐3 doublet.Conclusion. It is shown that type 1 IGF receptor is expressed in human articular cartilage and that the level of binding sites is significantly increased in OA chondrocytes. Interestingly, despite the higher level of binding sites in OA cells, no response to IGF‐1 stimulation was found in these cells. Our data suggest that this increase in specific binding may involve not only the type 1 IGF receptor but also IGFBP on the cell surface. The latter, by binding the IGF‐1, will diminish the bioavailability of IGF‐1 and thus prevent its anabolic action.