Transforming growth factor-beta (TGF-beta) 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-beta interaction with IGF-I on cell growth and differentiation in monolayer articular chondrocytes obtained from 5-week-old rats. TGF-beta (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-beta 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-beta on the IGF-I autocrine/paracrine axis was investigated. Administration of increasing concentrations of TGF-beta (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-beta also inhibited the production of a 41-kDa IGF-binding protein into the culture medium. Pretreatment with TGF-beta (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 beta-subunit was inhibited by simultaneous TGF-beta stimulation. These observations demonstrate that TGF-beta 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.