Recombinant bone morphogenetic protein-4, transforming growth factor-β₁, and activin A enhance the cartilage phenotype of articular chondrocytes in vitro

Bone morphogenetic protein−4 (BMP−4) is a member of the transforming growth factor-β (TGF-β) supergene family and is characterized by its ability to induce singly de novo cartilage and bone in vivo. The influence of rccombinant bone morphogenetic protein-4 and some related members, TGF-β₁, activin A, and inhibin A, on articular chondrocyte metabolism in the presence and absence of extracellular matrix has been examined. BMP-4 and TGF-β₁, stimulated [³⁶S]-sulfate incorporation in a dose-dependent manner in short-term monolayer, micromass, and explant cultures. Activin A showed a slight but significant stimulation of proteoglycan synthesis while inhibin A decreased metabolic activity. The effects observed were most pronounced in the explant culture system. Although the relative influence of the growth factors was less apparent in chondrocytes isolated from adult cartilage, the qualitative responses were similar with cells obtained from young animals. The maintenance and enhancement of the cartilage phenotype was further investigated by Northern blot analysis. BMP-4 and TGF-β₁, increased the levels of expression of type II collagen and proteoglycan aggrecan in short-term cultures, while activin A and inhibin A did not affect these parameters significantly when compared to serum-free control cultures. Binding experiments with ¹²⁵I-BMP-4, revealed the presence of specific, high-affinity binding sites with an apparent dissociation constant of 110 pM and about 6000 receptors per cell. Chemical cross-linking showed the presence of three components (apparent size 200, 90, and 7 kDa), demonstrating the presence of functional receptors for BMP-4 on primary articular chondrocytes.