The insulin-like growth factors (IGFs) I and II bind to articular cartilage via the IGF-binding proteins

Bovine articular cartilage discs (3 mm diameter x 400 mu m thick) were equilibrated in buffer containing I-125-insulin-like growth factor (IGF)-I (4 degrees C) +/- unlabeled IGF-I or IGF-II, Competition for binding to cartilage discs by each unlabeled IGF was concentration-dependent, with ED50 values for inhibition of I-125-IGF-I binding of 11 and 10 nM for IGF-I and -II, respectively, and saturation by 50 nM, By contrast, an analog of IGF-I with very low affinity for the insulin-like growth factor-binding proteins (IGF-BPs), des-(1-3)-IGF-I, was not competitive with I-125-IGF-I for cartilage binding even at 100-400 nM. Binding of the I-125-labeled IGF-II isoform to cartilage was competed for by unlabeled ICE-I or -II, with ED(50)s of 160 and 8 nM, respectively. This probably reflected the differential affinities of the endogenous IGF-BPs (IGF-BP-6 and -2) for IGF-II/IGF-I. Transport of I-125-IGF-I was also measured in an apparatus that allows diffusion only across the discs (400 mu m), by addition to one side and continuous monitoring of efflux on the other side. The time lag for transport of I-125-IGF was 266 min, an order of magnitude longer than the theoretical prediction for free diffusion in the matrix. I-125-IGF-I transport then reached a steady state rate (% efflux of total added I-125-IGF/unit time), which was subsequently accelerated similar to 2-fold by addition of an excess of unlabeled IGF-I, Taken together, these results indicate that IGF binding to cartilage, mostly through the IGF-BPs, regulates the transport of IC;Fs in articular cartilage, probably contributing to the control of their paracrine activities.