HUMAN OSTEOARTHRITIC CHONDROCYTES POSSESS AN INCREASED NUMBER OF INSULIN-LIKE GROWTH-FACTOR 1 BINDING-SITES BUT ARE UNRESPONSIVE TO ITS STIMULATION - POSSIBLE ROLE OF IGF-1-BINDING PROTEINS

Objective. To characterize the insulin-like growth factor 1 (IGF-1) re ceptor 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 t hese cells. Methods. Binding studies, Northern blot, immunohistochemic al analysis, and affinity cross-linking experiments were performed for characterization of the IGF receptor, and the latter method was also used for IGFBB determination. The biologic response was estimated via the incorporation of radiolabeled proline into a newly synthesized pro tein. 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 10(3) binding sites/cell. OA chondrocy tes had a lower affinity (K-d 15.4 +/- 4.7 nM) and a higher density (1 ,178.3 +/- 299.5 x 10(3) binding sites/cell) compared with normal cell s (P < 0.004 and P < 0.001, respectively). Immune-histochemical studie s with a monoclonal antibody (MAb) against the type 1 IGF receptor (al pha IR3) showed increased staining in OA cartilage compared with norma l tissue. Biologic responses of chondrocytes after IGF-1 stimulation r evealed that OA chondrocytes were unresponsive, whereas a 2.5-fold inc rease in new protein synthesis was observed in normal cells. Competiti on studies in normal chondrocytes revealed that both IGF-1 and IGF-2 d isplaced radiolabeled IGF-1 in a comparable manner; however, insulin a t high concentration weakly competes. Moreover, MAb alpha IR3 effectiv ely blocked specific binding in normal chondrocytes (77%), but not in OA chondrocytes (26%). Northern blot and covalent cross-linking analys es revealed the specific band characteristic of type 1 receptor. With the latter technique, other bands corresponding to the IGFBPs were als o detected. Comparison between normal and OA chondrocytes showed incre ased 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 tbe level of bindi ng 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 thi s increase in specific binding may involve not only the type 1 IGF rec eptor but also IGFBP on the cell surface. The latter, by binding the I GF-1, will diminish the bioavailability of IGF-1 and thus prevent its anabolic action.