Transforming growth factor beta 1 regulation of receptors and Smad3 expression according to incubation duration. (a) Human articular chondrocytes (HAC) were cultured as in Figure 1 and incubated with 5 ng/ml transforming growth factor beta 1 (TGFβ1) for different times. At the end of incubations, TGFβ receptor type I (TβRI), TGFβ receptor type II (TβRII), Smad3 and Smad7 mRNA levels were assayed by real-time RT-PCR. (b) In addition, TβRI, TβRII, Smad2/3 and phosphorylated Smad2/3 protein expression were analysed by western blot analysis. *, P < 0.05, **, P < 0.01, ***, P < 0.001.