Effect of Transforming Growth Factor-β on the Insulin-like Growth Factor-I Autocrine/Paracrine Axis in Cultured Rat Articular Chondrocytes

Abstract

Transforming growth factor-β (TGF-β) and insulin-like growth factor-I (IGF-I) are essential anabolic factors in articular cartilage. In this study, we concentrated on the elucidation of TGF-β interaction with IGF-I on cell growth and differentiation in monolayer articular chondrocytes obtained from 5-week-old rats. TGF-β (1 ng/ml) and IGF-I (25 ng/ml) stimulated DNA synthesis about 6.5- and 2.1-fold over control values, respectively. When TGF-β and IGF-I were added in combination, DNA synthesis was enhanced about 10.4-fold, indicating that the two peptides act in synergism. This synergistic action was also present in the expression of aggrecan mRNA. To study the mechanism of synergistic action, the effect of TGF-β on the IGF-I autocrine/paracrine axis was investigated. Administration of increasing concentrations of TGF-β (0.1-10 ng/ml) resulted in a dose-dependent decrease in medium IGF-I concentration that was reflected by decreased levels of IGF-I mRNA. TGF-β also inhibited the production of a 41-kDa IGF-binding protein into the culture medium. Pretreatment with TGF-β (1 ng/ml) for 12 h increased the binding of [125I]IGF-I to 140% of control by increasing the number of receptors without changes of affinity. Immunoprecipitation against phosphorylated tyrosine indicated that IGF-I-dependent autophosphorylation of IGF-I receptor β-subunit was inhibited by simultaneous TGF-β stimulation. These observations demonstrate that TGF-β acts synergistically with IGF-I and regulates the IGF-I autocrine/paracrine axis via a complex regulatory mechanism with decreased production of IGF-I and IGFBPs and dephosphorylation of IGF-I receptor, whereas there is an apparent up-regulation of the binding of [125I]IGF-I.