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.

Positive impact of IGF-1-coupled nanoparticles on the differentiation potential of human chondrocytes cultured on collagen scaffolds

“…8 In combination with TGF β, IGF-1 has a stimulatory effect on the adhesion of chondrocytes to fibronectin and collagen type II. 9 It is important to ensure the controlled delivery of biomolecules, like IGF-1, to avoid adverse effects. Nano-sized carriers (nanoparticles) are considered an attractive tool for this, as nanoparticles are small in dimension, offer a high surface area to volume ratio, and enable targeted drug delivery.…”

Novel strategies for enhancing tissue integration in cartilage repair

“…These data demonstrate a substrate dependent, significant increase in the migration o f chondrocytes isolated from young articular cartilage in response to TGFpi when coated onto fibronectin [P<0.005] (see fig.2.13a). As previously stated for IGF-1, TGFpi has also been shown to increase a 5 p i expression and in doing so increase adhesion to fibronectin (Loeser, 1997 Rosselot et al, 1994;Tsukazaki et al, 1994;Yaeger et al, 1997) and that TGFpi can increase the expression o f IGF-IRs by chondrocytes (Tsukazaki et al, 1994). A recent study demonstrated that this interaction occurs in situ during periosteal chondrogenesis (Fukumoto et al, 2003).…”

“…As interactions between fibronectin and chondrocytes is pi integrin dependent it is interesting to link research suggesting potential synergy between the IGF-1R and the p i integrin subunit by activation o f cell signalling proteins such as She (Shakibaei et al, 1999). IGF-1 has been shown to also increase levels o f the fibronectin specific receptor a 5 p l leading to increased adhesion (Loeser, 1997). No enhanced migration was seen with chondrocytes isolated from young articular cartilage seeded onto type II collagen or aggrecan (see fig.2.13a).…”

Cellular Physiology of Articular Cartilage in Health and Disease

“…The regulation of chondrocyte integrin function is important in the homeostasis of cartilage as well as in disease states in which interactions between chondrocytes and their ECM are altered. Factors that modulate chondrocyte ECM synthesis, such as IGF-1 and TGF-, also appear to modulate integrin-mediated attachment of chondrocytes to ECM proteins (Loeser 1994(Loeser , 1997. The effects of IGF-1 and TGF-on chondrocyte integrin expression and function, however, in vivo may depend on the relative levels of each growth factor present and thereby providing a means for sophisticated control of cell-matrix interactions in cartilage.…”