Fibronectin fragments upregulate insulin-like growth factor binding proteins in chondrocytes

Addition of fibronectin fragments (Fn-fs) to cultured cartilage explants has been shown to mediate extensive cartilage matrix degradation followed by anabolic responses.

To determine whether specific Fn-fs regulate cartilage metabolism through a mechanism, in part, involving insulin-like growth factor (IGF) and insulin-like growth factor binding proteins (IGFBPs).

Primary bovine articular chondrocyte cultures were treated with Fn-fs. mRNA from the cultures was analysed by Northern blotting. Changes in the levels of IGFBPs in cellular extracts and conditioned media were analysed by Western ligand blotting. Explant cultures of bovine articular cartilage were used to assay release of exogenous IGF-I and IGFBP-2. An analog of IGF-I with altered affinity for IGFBPs was used to assay the effect of IGFBPs on proteoglycan synthesis.

The Fn-fs increased protein levels of IGFBPs-2, -3 and -5 in conditioned media and of IGFBP-2 in cell extracts by as much as nine-fold. Conversely, the protein level of constitutively expressed IGBP-4 was decreased in conditioned medium. Northern blot analysis reflected increased IGFBP-3 mRNA but not decreased IGFBP-4 mRNA. The IGF-I analog was more effective at restoring PG synthesis suppression by Fn-fs than was wild type IGF-I.

The Fn-fs increased levels of IGFBPs in cultures of bovine articular chondrocytes and elicited release of IGFBP-2 and IGF-I from articular cartilage. The increased level of IGFBPs may trap IGF-I and account in part for the initial suppression of PG synthesis. Induced proteinases may subsequently liberate IGF-I and cause greatly enhanced anabolic processes, contributing to cartilage repair.