Growth factor regulation of chondrocyte integrins. Differential effects of insulin‐like growth factor 1 and transforming growth factor β on α1β1 integrin expression and chondrocyte adhesion to type VI collagen

Abstract

Objective. The ability of growth factors to modulate integrin expression may be important with regard to processes involved in tissue repair and remodeling. This study was undertaken to determine the effect of transforming growth factor β (TGFβ) and insulin‐like growth factor 1 (IGF‐1) on chondrocyte β1 integrin expression and integrin‐mediated adhesion to extracellular matrix proteins. Methods. Chondrocytes obtained from normal bovine articular cartilage were cultured in the presence or absence of 10% fetal bovine serum, 100 pM IGF‐1, or 100 pM TGF β. Integrin expression and function were measured by protein blotting of immunoprecipitated integrins, Northern blot analysis, and cell adhesion assays. Results. Immunoprecipitation with an anti–β1 integrin antibody coprecipitated the α1 integrin subunit and a band representing α3 and α5, as previously reported. Compared with serum‐free cultures, the use of serum resulted in an average 10‐fold increase in the α1 band and a 12‐fold increase in the α3/α5 band. IGF‐1 increased α1 and α3/α5 by an average of 3‐fold and 4‐fold, respectively. TGFβ also increased α3/α5 by >5‐fold but decreased α1 to an average of 24% of that found in serum‐free controls. Northern blot analysis revealed that TGFβ significantly increased α5 integrin subunit RNA levels. IGF‐1 did not have a significant effect on α5 or α1 integrin subunit RNA levels, suggesting that its effects on integrins are posttranscriptional. In cell adhesion assays, TGFβ treatment resulted in a 50% decrease in the adhesion of chondrocytes to type VI collagen, while adhesion to type II collagen and fibronectin was stimulated. IGF‐1 stimulated adhesion to all 3 proteins. An α1 integrin blocking antibody inhibited up to 75% of the adhesion of human chondrocytes to type VI collagen. Conclusion. Both IGF‐1 and TGFβ stimulate chondrocyte cell surface expression of the α3/α5 integrin subunit band and stimulate adhesion of chondrocytes to fibronectin and type II collagen. The 2 growth factors have opposite effects on expression of α1β1, with IGF‐1 increasing and TGFβ decreasing cell surface levels of this integrin. TGFβ‐treated cells also have decreased adhesion to type VI collagen. The opposing effects of IGF‐1 and TGFβ on chondrocyte expression of α1/β1 and on adhesion to type VI collagen suggest that α1β1 mediates chondrocyte adhesion to type VI collagen. This was confirmed by using an antibody to the α1 integrin subunit to block adhesion of chondrocytes to type VI collagen.