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

Abstract

Bovine articular cartilage discs (3 mm diameter x 400 μm thick) were equilibrated in buffer containing 125I- insulin-like growth factor (IGF)-I (4 °C) ± unlabeled IGF-I or IGF-H. Competition for binding to cartilage discs by each unlabeled IGF was concentration-dependent, with ED50 values for inhibition of 125I-IGF-I binding of 11 and 10 nM for IGF-I and -H, 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 125I-IGF-I for cartilage binding even at 100-400 nM. Binding of the 125I-labeled IGF-II isoform to cartilage was competed for by unlabeled IGF-I or -II, with ED50s of 160 and 8 nM, respectively. This probably reflected the differential affinities of the endogenous IGF-BPs (IGFBP-6 and -2) for IGF-II/IGF-I. Transport of 125I-IGF-I was also measured in an apparatus that allows diffusion only across the discs (400 μm), by addition to one side and continuous monitoring of efflux on the other side. The time lag for transport of 125I-IGF was 266 min, an order of magnitude longer than the theoretical prediction for free diffusion in the matrix. 125I-IGF-I transport then reached a steady state rate (% efflux of total added 125I-IGF/unit time), which was subsequently accelerated ~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 IGFs in articular cartilage, probably contributing to the control of their paracrine activities.