Effects of fibronectin on articular cartilage chondrocyte proteoglycan synthesis and response to insulin-like growth factor-I

Fibronectin, a ubiquitous glycoprotein of the extracellular matrix, serves as a substrate for cell attachment. Binding to fibronectin through cell-surface receptors promotes a flattened cell shape, stimulates the phosphorylation of intracellular protein, and changes the pattern of gene expression. Although fibronectin is abundant in normal articular cartilage, its effects on chondrocytes are not well understood. Proteolytic fragments of fibronectin stimulate the catabolism of matrix in articular cartilage and may promote the degeneration of cartilage in osteoarthritis; however, intact fibronectin may regulate other aspects of matrix metabolism, including matrix synthesis. To determine whether intact fibronectin affects the synthetic activity of chondrocytes, as well as to determine the responses of chondrocytes to the anabolic growth factor insulin-like growth factor-I, we compared the incorporation of [S-35] by articular chondrocytes of the rat cultured in the presence and absence of commercially prepared cellular fibronectin and 0, 10, or 100 ng/ml recombinant human insulin-like growth factor-I. Monolayer and alginate suspension cultures were compared to determine whether responses differed under conditions in which fibronectin promoted a flattened cell shape (monolayer culture) and under those in which cells maintained a spherical shape (alginate culture). In alginate cultures, fibronectin alone stimulated the incorporation of [S-35]. Fibronectin with 10 ng/ml insulin-like growth factor-I had additive effects in alginate culture, producing the maximum incorporation of [S-35]. In monolayer cultures, fibronectin did not stimulate incorporation and blocked stimulation by 100 ng/ml insulin-like growth factor-I. The cells from the monolayer culture were much less active synthetically (at all doses of the growth factor) than those cultured in alginate. Thus, fibronectin enhanced proteoglycan synthesis and the response to insulin-like growth factor-I in alginate but inhibited the response to the growth factor in monolayers. These observations suggest intact fibronectin may contribute to the maintenance or repair of the matrix of articular cartilage by stimulating proteoglycan synthesis.