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 equi librated in buffer containing I-125-insulin-like growth factor (IGF)-I (4 d egrees C) +/- unlabeled IGF-I or IGF-II, Competition for binding to cartila ge discs by each unlabeled IGF was concentration-dependent, with ED50 value s 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 a nd 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 ac ross the discs (400 mu m), by addition to one side and continuous monitorin g 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 f ree diffusion in the matrix. I-125-IGF-I transport then reached a steady st ate rate (% efflux of total added I-125-IGF/unit time), which was subsequen tly accelerated similar to 2-fold by addition of an excess of unlabeled IGF -I, Taken together, these results indicate that IGF binding to cartilage, m ostly through the IGF-BPs, regulates the transport of IC;Fs in articular ca rtilage, probably contributing to the control of their paracrine activities .