EFFECT OF TRANSFORMING GROWTH-FACTOR-BETA ON THE INSULIN-LIKE GROWTH-FACTOR-I AUTOCRINE PARACRINE AXIS IN CULTURED RAT ARTICULAR CHONDROCYTES/

Transforming growth factor-beta (TGF-beta) and insulinlike growth fact or-I (IGF-I) are essential anabolic factors in articular cartilage, In this study, we concentrated on the elucidation of TGF-beta interactio n with IGF-I on cell growth and differentiation in monolayer articular chondrocytes obtained from B-week-old rats. TGF-beta (1 ng/ml) and IG F-I (25 ng/ml) stimulated DNA synthesis about 6.5- and 2.1-fold over c ontrol values, respectively. When TGF-beta and IGF-I were added in com bination, DNA synthesis was enhanced about 10.4-fold, indicating that the two peptides act in synergism. This synergistic action was also pr esent in the expression of aggrecan mRNA. To study the mechanism of sy nergistic action, the effect of TGF-beta on the IGF-I autocrine/paracr ine axis was investigated. Administration of increasing concentrations of TGF-beta (0.1-10 ng/ml) resulted in a dose-dependent decrease in m edium IGF-I concentration that was reflected by decreased levels of IG F-I mRNA. TGF-beta also inhibited the production of a 41-kDa IGF-bindi ng protein into the culture medium. Pretreatment with TGF-beta (1 ng/m l) for 12 h increased the binding of [I-125]IGF-I to 140% of control b y increasing the number of receptors without changes of affinity. Immu noprecipitation against phosphorylated tyrosine indicated that IGF-I-d ependent autophosphorylation of IGF-I receptor beta-subunit was inhibi ted by simultaneous TGF-beta stimulation. These observations demonstra te that TGF-beta acts synergistically with IGF-I and regulates the IGF -I autocrine/paracrine axis via a complex regulatory mechanism with de creased production of IGF-I and IGFBPs and dephosphorylation of IGF-I receptor, whereas there is an apparent up-regulation of the binding of [I-125]IGF-I. (c) 1994 Academic Press, Inc.