Transforming growth factor beta (TGF-beta) is a multipotent regulator of cell proliferation and extracellular matrix production. The effect of TGF-beta on chondrocyte matrix production was studied in relation to the expression of TGF-beta binding proteins. The effect of TGF-beta on proteoglycan synthesis of isolated articular chondrocytes depended on the culture period. Proteoglycan synthesis of chondrocytes which were cultured for one day was inhibited by TGF-beta whereas proteoglycan synthesis of chondrocytes cultured in monolayer for seven days or longer was stimulated by TGF-beta. To investigate if this differential response is related to a distinct expression of TGF-beta receptors, this parameter was studied by affinity labelling.
Chondrocytes were incubated with 100 pM TGF-beta labelled with iodine-125. Crosslinking was performed using 0.25 mM disuccinimidyl suberate. Membrane proteins were extracted and analysed by denaturating sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography.
Freshly isolated and cultured chondrocytes expressed types I, II, and III TGF-beta receptors. The type II TGF-beta receptor of cultured chondrocytes appeared to be about 15 kilodaltons smaller than the type II TGF-beta receptor expressed on freshly isolated chondrocytes, however.
As the type II TGF-beta receptors appears to be involved in signal transduction, this change in size of the type II TGF-beta receptor might be related to the differential effect of TGF-beta on proteoglycan synthesis of freshly isolated and cultured bovine articular chondrocytes.