Growth factor regulation of chondrocyte integrins. Differential effects of insulin-like growth factor 1 and transforming growth factor beta on alpha 1 beta 1 integrin expression and chondrocyte adhesion to type VI collagen.
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 beta (TGF beta) and insulin-like growth factor 1 (IGF-1) on chondrocyte beta 1 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 beta. Integrin expression and function were measured by protein blotting of immunoprecipitated integrins, Northern blot analysis, and cell adhesion assays.


RESULTS
Immunoprecipitation with an anti-beta 1 integrin antibody coprecipitated the alpha 1 integrin subunit and a band representing alpha 3 and alpha 5, as previously reported. Compared with serum-free cultures, the use of serum resulted in an average 10-fold increase in the alpha 1 band and a 12-fold increase in the alpha 3/alpha 5 band. IGF-1 increased alpha 1 and alpha 3/alpha 5 by an average of 3-fold and 4-fold, respectively. TGF beta also increased alpha 3/alpha 5 by > 5-fold but decreased alpha 1 to an average of 24% of that found in serum-free controls. Northern blot analysis revealed that TGF beta significantly increased alpha 5 integrin subunit RNA levels. IGF-1 did not have a significant effect on alpha 5 or alpha 1 integrin subunit RNA levels, suggesting that its effects on integrins are posttranscriptional. In cell adhesion assays, TGF beta 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 alpha 1 integrin blocking antibody inhibited up to 75% of the adhesion of human chondrocytes to type VI collagen.


CONCLUSIONS
Both IGF-1 and TGF beta stimulate chondrocyte cell surface expression of the alpha 3/alpha 5 integrin subunit band and stimulate adhesion of chondrocytes to fibronectin and type II collagen. The 2 growth factors have opposite effects on expression of alpha 1 beta 1, with IGF-1 increasing and TGF beta decreasing cell surface levels of this integrin. TGF beta-treated cells also have decreased adhesion to type VI collagen. The opposing effects of IGF-1 and TGF beta on chondrocyte expression of alpha 1 beta 1 and on adhesion to type VI collagen suggest that alpha 1 beta 1 mediates chondrocyte adhesion to type VI collagen. This was confirmed by using an antibody to the alpha 1 integrin subunit to block adhesion of chondrocytes to type VI collagen.
Loeser
R F
RF
Bowman Gray School of Medicine, Wake Forest University, Department of Internal Medicine-Rheumatology, Winston-Salem, NC 27157, USA.
D000818:Animals; D015152:Blotting, Northern; D002358:Cartilage, Articular / Q000737:chemistry* / Q000166:cytology*; D002417:Cattle; D002478:Cells, Cultured; D016895:Culture Media, Serum-Free; D006133:Growth Substances / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D016023:Integrins / Q000096:biosynthesis* / Q000235:genetics; D011233:Precipitin Tests; D012333:RNA, Messenger / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000494:pharmacology
D016895:Culture Media, Serum-Free; D006133:Growth Substances; D016023:Integrins; D012333:RNA, Messenger; D016212:Transforming Growth Factor beta; D007334:Insulin-Like Growth Factor I